He was one of the 12 sons of Cennétig mac Lorcáin (d. 951), king of Dál gCais and king of Tuadmumu (Thomond), modern County Clare, then a sub-kingdom in the north of Munster. Cennétig was described as rígdamna Caisil, meaning that he was either heir or candidate ("king material") to the kingship of Cashel or Munster, although this might be a later interpolation. Brian's mother was Bé Binn inion Urchadh, daughter of Urchadh mac Murchadh (d. 945), king of Maigh Seóla in west Connacht. That they belonged to the Uí Briúin Seóla may explain why he received the name Brian, which was rare among the Dál gCais.
Brian's family were descended from the Ui Tairdelbach branch of the Dal gCais (or Deis Tuisceart). This branch had recently taken power from the more powerful Ui Óengusso branch which had traditionally supplied the Kings of the Dal gCais. This power shift occurred after the death of Ui Óengusso King Rebechan Mac Mothla who died as King of the Dal gCais in 934. The sons of Brian's grandfather Lorcan seized the opportunity and took power from the rival branch, with Brian's father Cennétig being the most successful of these. His father was the first King of the Dal gCais to lead an army beyond his own territory and lead an expedition as far north as Athlone. By his death in 951 had been acknowledged as "King of Tuadmumu".  His brother Mahon built on these achievements and was the first to capture Cashel and become King of Munster.
Brian was born at Kincora, Killaloe, a town in the region of Tuadmumu. Brian's posthumous cognomen "Bóruma" (anglicised as Boru) may have referred to "Béal Bóruma", a fort north of Killaloe, where the Dál gCais held sway. Another explanation, though possibly a late (re-)interpretation, is that the nickname represented Old Irish bóruma "of the cattle tribute", referring to his capacity as a powerful overlord.
The River Shannon served as an easy route by which raids could be made against the provinces of Connacht and Meath. Both Brian's father, Cennétig mac Lorcáin, and his older brother Mathgamain conducted river-borne raids, in which the young Brian would undoubtedly have participated. This was probably the root of his appreciation for naval forces in his later career. Thus an important influence upon the Dalcassians was the presence of the Hiberno-Norse city of Limerick on an isthmus around which the Shannon River winds (known today as King's Island or the Island Field). The Norse had made many a raid themselves from the Shannon, and the Dalcassians likely benefited from some interaction with them, from which they would have been exposed to innovations such as superior weapons and ship design, all factors that may have contributed to their growing power.
When their father died, the kingship of Tuadmumu passed to Brian's older brother, Mathgamain, and, when Mathgamain was killed in 976, Brian replaced him. Subsequently, he became the king of the entire kingdom of Munster.
Reign of his brother, Mathgamain
In 964, Brian's older brother, Mathgamain, claimed control over the entire province of Munster by capturing the Rock of Cashel, capital of the ancient Eóganachta, the hereditary overlords or High Kings of Munster, but who in dynastic strife and with multiple assassinations had weakened themselves to the point they were now impotent. Earlier attacks from both the Uí Néill and Vikings were also factors. This situation allowed the illegitimate (from the Eóganacht perspective) but militarized Dál Cais to attempt to seize the provincial kingship. Mathgamain was never fully recognized and was opposed throughout his career in the 960s and 970s by Máel Muad mac Brain, a semi-outsider from the Cashel perspective but still a legitimate Eóganacht claimant from far south Munster.
In addition to Máel Muad, the Norse king Ivar of Limerick was a threat and may have been attempting to establish some overlordship in the province or a region of it himself, with the Cogad Gaedel re Gallaib even asserting he actually achieved this until routed by Mathgamain in the celebrated Battle of Sulcoit in 967. This victory was not decisive and eventually there grew up a brief alliance of sorts between Mathgamain, Máel Muad and others to drive the Norse "soldiers" or "officials" out of Munster and destroy their Limerick fortress in 972. The two Gaelic claimants were soon back to fighting and the fortuitous capture of Mathgamain in 976 by Donnubán mac Cathail allowed him to be effortlessly dispatched or murdered by Máel Muad, who would now rule as king of Cashel for two years.
The Dál Cais remained a powerful force and Brian quickly proved to be as fine a commander of armies as his brother. After first dispatching the already much weakened Ivar in 977, he challenged Máel Muad in 978 and defeated him in the fateful Battle of Belach Lechta, after which all the Eóganachta were no longer viable at the provincial level and Brian and the Dál Cais now enjoyed the overlordship, although not the traditional kingship of the province, which was based on lineage. Either soon before or soon after his victory over Máel Muad, Brian routed Donnubán and the remainder of the Norse army in the Battle of Cathair Cuan, there probably slaying the last of Ivar's sons and successor Aralt. He then allowed some of the Norse to remain in their settlement, but they were wealthy and now central to trade in the region, with a fleet of great value.
Cian, the son of his brother Mathgamain's sworn enemy Máel Muad, later became a loyal ally of Brian and served under him in a number of campaigns.
Banner reputedly used by Brian 
of which the Clare GAA colors are based.
Having established unchallenged rule over his home Province of Munster, Brian turned to extending his authority over the neighboring provinces of Leinster to the east and Connacht to the north. By doing so, he came into conflict with High King Máel Sechnaill mac Domnaill whose power base was the Province of Meath.
For the next fifteen years, from 982 to 997, High King Máel Sechnaill repeatedly led armies into Leinster and Munster, while Brian, like his father and brother before him, led his naval forces up the Shannon to attack Connacht and Meath on either side of the river. He suffered quite a few reverses in this struggle, but appears to have learned from his setbacks. He developed a military strategy that would serve him well throughout his career: the coordinated use of forces on both land and water, including on rivers and along Ireland's coast. Brian's naval forces, which included contingents supplied by the Hiberno-Norse cities that he brought under his control, provided both indirect and direct support for his forces on land. Indirect support involved a fleet making a diversionary attack on an enemy in a location far away from where Brian planned to strike with his army. Direct support involved naval forces acting as one arm in a strategic pincer, the army forming the other arm.
In 996 Brian finally managed to control the province of Leinster, which may have been what led Máel Sechnaill to reach a compromise with him in the following year. By recognising Brian's authority over Leth Moga, that is, the Southern Half, which included the Provinces of Munster and Leinster (and the Hiberno-Norse cities within them), Máel Sechnaill was simply accepting the reality that confronted him and retained control over Leth Cuinn, that is, the Northern Half, which consisted of the Provinces of Meath, Connacht, and Ulster.
Precisely because he had submitted to Brian's authority, the King of Leinster was overthrown in 998 and replaced by Máel Mórda mac Murchada. Given the circumstances under which Máel Mórda had been appointed, it is not surprising that he launched an open rebellion against Brian's authority.
In response, Brian assembled the forces of the Province of Munster with the intention of laying siege to the Hiberno-Norse city of Dublin, which was ruled by Máel Mórda's ally and cousin, Sigtrygg Silkbeard. Together Máel Mórda and Sigtrygg determined to meet Brian's army in battle rather than risk a siege. Thus, in 999, the opposing armies fought the Battle of Glenmama. The Irish annals all agree that this was a particularly fierce and bloody engagement, although claims that it lasted from morning until midnight, or that the combined Leinster-Dublin force lost 4,000 killed are open to question. In any case, Brian followed up his victory, as he and his brother had in the aftermath of the Battle of Sulcoit thirty-two years before, by capturing and sacking the enemy's city.
Once again Brian opted for reconciliation; he requested Sigtrygg to return and resume his position as ruler of Dublin, giving Sigtrygg the hand of one of his daughters in marriage, just as he had with the Eoganacht King, Cian. It may have been on this occasion that Brian married Sigtrygg's mother and Máel Mórda's sister Gormflaith, the former wife of Máel Sechnaill.
The struggle for Ireland
Brian made it clear that his ambitions had not been satisfied by the compromise of 997 when, in the year 1000, he led a combined Munster-Leinster-Dublin army in an attack on High King Máel Sechnaill mac Domnaill's home province of Meath. The struggle over who would control all of Ireland was renewed. Máel Sechnaill's most important ally was the King of Connacht, Cathal mac Conchobar mac Taidg (O'Connor), but this presented a number of problems. The provinces of Meath and Connacht were separated by the Shannon River, which served as both a route by which Brian's naval forces could attack the shores of either province and as a barrier to the two rulers providing mutual support for each other. Máel Sechnaill came up with an ingenious solution; two bridges would be erected across the Shannon. These bridges would serve as both obstacles preventing Brian's fleet from traveling up the Shannon and a means by which the armies of the provinces of Meath and Connacht could cross over into each other's kingdoms.
The Annals state that, in the year 1002, Máel Sechnaill surrendered his title to Brian, although they do not say anything about how or why this came about. The Cogadh Gaedhil re Gallaibh provides a story in which Brian challenges High King Máel Sechnaill to a battle at the Hill of Tara in the province of Meath, but the High King requests a month-long truce so that he can mobilise his forces, which Brian grants him. Máel Sechnaill fails to rally the regional rulers who are nominally his subordinates by the time the deadline arrives, and he is forced to surrender his title to Brian.
There have been some doubts expressed about this explanation, given Brian's style of engaging in war; if he had found his opponent at a disadvantage it is most likely he would have taken full advantage of it rather than allowing his enemy the time to even the odds. Also, given the length and intensity of the struggle between Máel Sechnaill and Brian, it seems unlikely that the High King would surrender his title without a fight. It is generally accepted that in 1002 Brian became the new High King of Ireland.
Unlike some who had previously held the title, Brian intended to be High King in more than name. To accomplish this he needed to impose his will upon the regional rulers of the only province that did not already recognise his authority, Ulster.
Ulster's geography presented a formidable challenge. There were three main routes by which an invading army could enter the province, and all three favoured the defenders. Brian first had to find a means of getting through or around these defensive 'choke points', and then he had to subdue the fiercely independent regional Kings of Ulster. It took Brian ten years of campaigning to achieve his goal, which, considering that he could and did call on all of the military forces of the rest of Ireland, indicates how formidable the Kings of Ulster were. Once again, it was his coordinated use of forces on land and at sea that allowed him to triumph; while the rulers of Ulster could bring the advance of Brian's army to a halt, they could not prevent his fleet from attacking the shores of their kingdoms. Once Brian entered the province of Ulster, he systematically defeated each of the regional rulers who defied him, forcing them to recognise him as their overlord.
It was during this process that Brian pursued an alternative means of consolidating his control, not merely over the province of Ulster, but over Ireland as a whole. In contrast to its structure elsewhere, the Church in Ireland was centred, not around the bishops of dioceses and archbishops of archdioceses, but rather around monasteries headed by powerful abbots who were members of the royal dynasties of the lands in which their monasteries resided. Among the most important monasteries was Armagh, located in the Province of Ulster.
Brian's advisor, Maelsuthain O'Carroll, documented in the 'Book of Armagh' that, in the year 1005, Brian donated 22 ounces of gold to this monastery and declared that Armagh was the religious capital of Ireland, to which all other monasteries should send the funds they collected. This was a clever move, for the supremacy of the monastery of Armagh would last only so long as Brian remained the High King. Therefore, it was in the interest of Armagh to support Brian with all their wealth and power.
Brian is not referred to in the passage from the 'Book of Armagh' as the 'Ard Rí' —that is, High-King— but rather he is declared "Imperator Scottorum," or "Emperor of the Scots."
Though it is only speculation, it has been suggested that Brian and the Church in Ireland were together seeking to establish a new form of kingship in Ireland, one that was modelled after the kingships of England and France, in which there were no lesser ranks of regional kings – simply one king who had (or sought to have) power over all in a unitary state. In any case, whether as High King or Emperor, by 1011 all of the regional rulers in Ireland acknowledged Brian's authority. No sooner had this been achieved than it was lost again.
Máel Mórda mac Murchada of Leinster had only accepted Brian's authority grudgingly, and in 1012 he rose in rebellion. The Cogadh Gaedhil re Gallaibh relates a story in which one of Brian's sons insults Máel Mórda, which leads him to declare his independence from Brian's authority. Whatever the actual reason was, Máel Mórda sought allies with which to defy the High-King. He found one in a regional ruler in Ulster who had only recently submitted to Brian. Together they attacked the province of Meath, where the former High King Máel Sechnaill sought Brian's help to defend his kingdom.
In 1013 Brian led a force from his own province of Munster and from southern Connacht into Leinster; a detachment under his son Murchad ravaged the southern half of the province of Leinster for three months. The forces under Murchad and Brian were reunited on 9 September outside the walls of Dublin. The city was blockaded, but it was the High King's army that ran out of supplies first, so that Brian was forced to abandon the siege and return to Munster around Christmas.
Battle of Clontarf
Máel Mórda was aware that the High King would return to Dublin in 1014 to try once more to defeat him. He may have hoped that by defying Brian, he could enlist the aid of all the other regional rulers. If so, he was sorely disappointed. The province of Ulster and most of the province of Connacht failed to support either side of the conflict, with the exception of a single ruler in Ulster who sent troops to Máel Mórda. His inability to obtain troops from any rulers in Ireland, may explain why Máel Mórda sought support from rulers outside Ireland. He instructed his subordinate and cousin, Sigtrygg, the ruler of Dublin, to travel overseas to enlist aid.
Sigtrygg sailed to Orkney, and on his return stopped at the Isle of Man. These islands had been occupied by the Vikings long before and the Hiberno-Norse had close ties with Orkney and the Isle of Man. There was even a precedent for employing Norsemen from the isles; they had been used by Sigtrygg's father, Amlaíb Cuarán, in 980, and by Sigtrygg himself in 990. Their incentive was loot, not land.
Contrary to the assertions made in the Cogadh Gaedhil re Gallaibh, this was not an attempt by the Vikings to reconquer Ireland. All of the Norsemen, both the Norse-Gaels of Dublin and the Norsemen from the Isles, were in the service of Máel Mórda. The High King had 'Vikings' in his army as well: the Hiberno-Norse of Limerick and probably those of Waterford, Wexford, and Cork as well. Some sources include a rival gang of Norse mercenaries from the Isle of Man. Essentially this conflict was an Irish civil war with minor foreign participation.
Along with whatever troops he obtained from abroad, Brian mustered troops from his home province of Munster, southern Connacht, and the province of Meath, commanded by his old rival Máel Sechnaill mac Domnaill. Brian's army may have outnumbered Máel Mórda's, since Brian felt secure enough to dispatch a mounted detachment under the command of his youngest son, Donnchad, to raid southern Leinster, presumably hoping to force Máel Mórda to release his contingents from there to return to defend their homes.
A disagreement with the King of Meath resulted in Máel Sechnaill withdrawing his support. Brian sent a messenger to find Donnchad and ask him to return with his detachment, but the call for help came too late. To compound Brian's problems, the Norse contingents, led by , Earl of Orkney and Brodir of the Isle of Man, arrived on Palm Sunday, 18 April. The battle occurred five days later, on Good Friday, 23 April 1014 just north of the city of Dublin, at Clontarf.
Brian on the Morning of Clontarf
Plaque at Brian Boru's burial place in St. Patrick's Cathedral, (COI), Armagh
Brian being killed by Brodir
All of the accounts state that the Battle of Clontarf lasted all day. This may be an exaggeration, or it may suggest a long-drawn-out fight.
There are many accounts of how Brian was killed, from dying in heroic man-to-man combat to being killed by the fleeing Viking mercenary Brodir while praying in his tent at Clontarf. After his death, his body was taken to Swords, Co. Dublin for the wake and then to Armagh to be buried. His tomb is said to be in the north wall of St Patrick's Cathedral in the city of Armagh.