Origins and antecedents
Prehistoric and legendary ancestors
During the past 12,500 years of inhabitation, Ireland has witnessed some different peoples arrive on its shores. The ancient peoples of Ireland—such as the creators of the Céide Fields and Newgrange—are almost unknown. Neither their languages nor the terms they used to describe themselves have survived. As late as the middle centuries of the 1st millennium the inhabitants of Ireland did not appear to have a collective name for themselves.
Ireland itself was known by a number of different names, including Banba, Fódla, Ériu by the islanders, Iouerne and Hiverne to the Greeks, and Hibernia to the Romans.
Scotland takes its name from Scota, who in Irish mythology, Scottish mythology, and pseudohistory, is the name given to two different mythological daughters of two different Egyptian Pharaohs to whom the Gaels traced their ancestry, allegedly explaining the name Scoti, applied by the Romans to Irish raiders, and later to the Irish invaders of Argyll and Caledonia which became known as Scotland. Other Latin names for people from Ireland in Classic and Mediaeval sources include Attacotti and Gael. This last word, derived from the Welsh gwyddel "raiders", was eventually adopted by the Irish for themselves. However, as a term it is on a par with Viking, as it describes an activity (raiding, piracy) and its proponents, not their actual ethnic affiliations.
The terms Irish and Ireland are probably derived from the goddess Ériu. A variety of historical ethnic groups have inhabited the island, including the Airgialla, Fir Ol nEchmacht, Delbhna, Fir Bolg, Érainn, Eóganachta, Mairtine, Conmaicne, Soghain, and Ulaid. In the cases of the Conmaicne, Delbhna, and perhaps Érainn, it can be demonstrated that the tribe took their name from their chief deity, or in the case of the Ciannachta, Eóganachta, and possibly the Soghain, a deified ancestor. This practice is paralleled by the Anglo-Saxon dynasties' claims of descent from Woden, via his sons Wecta, Baeldaeg, Casere and Wihtlaeg.
The Greek mythographer Euhemerus originated the concept of Euhemerism, which treats mythological accounts as a reflection of actual historical events shaped by retelling and traditional mores. In the 12th century, Icelandic bard and historian Snorri Sturluson proposed that the Norse gods were originally historical war leaders and kings, who later became cult figures, eventually set into society as gods. This view is in agreement with Irish historians such as T. F. O'Rahilly and Francis John Byrne; the early chapters of their respective books, Early Irish history and mythology (reprinted 2004) and Irish Kings and High-Kings (3rd revised edition, 2001), deal in depth with the origins and status of many Irish ancestral deities.
One legend states that the Irish were descended from one Míl Espáine, whose sons supposedly conquered Ireland around 1000 BC or later. The character is almost certainly a mere personification of a supposed migration by a group or groups from Iberia to Ireland. It is from this that the Irish were, as late as the 1800s, popularly known as "Milesian". Medieval Irish historians, over the course of several centuries, created the genealogical dogma that all Irish were descendants of Míl, ignoring the fact that their own works demonstrated inhabitants in Ireland prior to his supposed arrival.
This doctrine was adapted between the 10th and 12th centuries, as demonstrated in the works of Eochaidh Ua Floinn (936–1004); Flann Mainistrech (died 25 November 1056); Tanaide (died c. 1075) and Gilla Cómáin mac Gilla Samthainde (fl. 1072). Many of their compositions were incorporated into the compendium Lebor Gabála Érenn.
This tradition was enhanced and embedded in the tradition by successive historians such as Dubsúilech Ó Maolconaire (died 1270); Seán Mór Ó Dubhagáin (d.1372); Giolla Íosa Mór Mac Fir Bhisigh (fl. 1390–1418); Pilip Ballach Ó Duibhgeannáin (fl. 1579–1590) and Flann Mac Aodhagáin (alive 1640). The first Irish historian who questioned the reliability of such accounts was Dubhaltach Mac Fhirbhisigh (murdered 1671).
Genetic research shows a strong similarity between the Y chromosome haplotypes of Irish men with Gaelic surnames and males from the area of Spain and Portugal, especially Galicia, Asturias, and Cantabria (and perhaps former Basque country). The incidence of the R1b haplogroup is 70% or more in Celtic regions – Cumbria and Cornwall in England, the Celtic Northern region in Portugal (Douro Litoral, Minho and Trás-os-Montes e Alto Douro), northern Spain (Celtic Galicia, Asturias, León, Cantabria and Basque Country), western France (Béarn, Gascony, Guyenne, Saintonge, Angoumois, Aunis, Poitou, Touraine, Anjou and the Celtic Brittany), and Celtic Countries – Wales and Scotland in Britain. R1b's incidence declines gradually with distance from these areas but it is still common across the central areas of Europe. R1b is the most frequent haplogroup in Germany and in the Low Countries, and is common in southern Scandinavia and in northern and central Italy. This led writers, such as Stephen Oppenheimer and Bryan Sykes, to conclude that the majority of Irish people primarily descend from an "Iberian refugium" population bottleneck dating back to the last ice age.
However, this haplogroup is now believed by some to have originated over 12,000 years more recently than previously thought. It thus follows that Irish and many other R1b subclades would be considerably younger than the maximum age of 18,000 years. The previous estimates, based on inaccurate dating methods (30,000+ years BP), made R1b and its subclades seem to be more useful indicators of the paleolithic era populations of western Europe than they actually are. According to recent 2009 studies by Bramanti et al. and Malmström et al. on mtDNA, related western European populations appear to be largely from the neolithic and not paleolithic era, as previously thought. There was discontinuity between mesolithic central Europe and modern European populations mainly due to an extremely high frequency of haplogroup U (particularly U5) types in mesolithic central European sites.
The existence of an especially strong genetic association between the Irish and the Basques, one even closer than the relationship between other west Europeans, was first challenged in 2005, and in 2007 scientists began looking at the possibility of a more recent Mesolithic- or even Neolithic-era entrance of R1b into Europe. A new study published in 2010 by Balaresque et al. implies either a Mesolithic- or Neolithic- (not Paleolithic-) era entrance of R1b into Europe. However, all these genetic studies agree that the Irish and Basque (along with the Welsh) share the highest percentage of R1b populations.
A recent whole genome analysis of 1 neolithic and 3 Bronze Age skeletal remains in Ireland suggested that the original farming population was mostly similar to present-day Sardinians, and the 3 Bronze Age remains had a Steppe component to their genetics showing links with Eastern Europe. Most modern Irish share more DNA with the 3 Bronze Age men from Rathlin than with the earlier Ballynahatty neolithic woman.
A recent genetic study done on the Irish show that they have two main ancestral sources: a French component (mostly northwestern French) and West Norwegian from the Viking era. 
Black Irish is an ambiguous term sometimes used (mainly outside Ireland) as a reference to a dark-haired phenotype appearing in people of Irish origin. However, dark hair in people of Irish descent is common, although darker skin complexions appear less frequently. One popular speculation suggests the Black Irish are descendants of survivors of the Spanish Armada, despite research discrediting such claims. In Bob Quinn's documentary series Atlantean, he explores an alternative 'Iberian' hypothesis, proposing the existence of an ancient sea-trading route skirting the Atlantic coast from North Africa and the Iberian Peninsula to regions such as Connemara. While preferring the term "The Atlantean Irish", Quinn's reference to certain phenotypical characteristics (within elements of the Irish populace and diaspora) as possible evidence of a previous Hibernian-Iberian (and possibly Berber) admixture mirrors common descriptions of the Black Irish.
The term has also come to be used to refer to the African-Irish descendants of those who live on Barbados and Montserrat, a number of whom have Irish surnames, still retain part of their Irish accents and sing bilingual songs.