The county is rich in evidence of early human habitation. Portal tombs (sometimes called dolmens) exist at Ballybrittas (on Bree Hill) and at Newbawn — and date from the Neolithic period or earlier. Remains from the Bronze Age period are far more widespread. Early Irish tribes formed the Kingdom of Uí Cheinnsealaig, an area that was slightly larger than the current County Wexford.
County Wexford was one of the earliest areas of Ireland to be Christianised, in the early 5th century. Later, from 819 onwards, the Vikings invaded and plundered many Christian sites in the county. Vikings settled at Wexford town near the end of the 9th century.
In 1169, Wexford was the site of the invasion of Ireland by Normans at the behest of Diarmuid Mac Murrough, King of Uí Cheinnsealaig and king of Leinster (Laigin). This was followed by the subsequent colonisation of the country by the Anglo-Normans.
The native Irish began to regain some of their former territories in the 14th century, especially in the north of the county, principally under Art MacMurrough Kavanagh. Under Henry VIII, the great religious houses were dissolved, 1536–41; in County Wexford this included Glascarrig Priory, Clonmines Priory, Tintern Abbey, and Dunbrody Abbey.
On 23 October 1641, a major rebellion broke out in Ireland, and County Wexford produced strong support for Confederate Ireland. Oliver Cromwell and his English Parliamentarian Army arrived in 1649 in the county and captured it. The lands of the Irish and Anglo-Normans were confiscated and given to Cromwell's soldiers as payment for their service in the Parliamentarian Army. At Duncannon, in the south-west of the county, James II, after his defeat at the Battle of the Boyne, embarked for Kinsale and then to exile in France.
County Wexford was the most important area in which the Irish Rebellion of 1798 was fought, during which significant battles occurred at The Battle of Oulart Hill during the 1798 rebellion. Vinegar Hill (Enniscorthy) and New Ross. The famous ballad "Boolavogue" was written in remembrance of the Wexford Rising. At Easter 1916, a small rebellion occurred at Enniscorthy town, on cue with that in Dublin. During World War II, German planes bombed Campile. In 1963 John F. Kennedy, then President of the United States, visited the county and his ancestral home at Dunganstown, near New Ross.