Slavery had already existed in Ireland for centuries by the time the Vikings began to establish their coastal settlements, but it was under the Norse-Gael Kingdom of Dublin that it reached its peak. Gaelic raiders kidnapped and enslaved people from across the Irish Sea for two centuries after the Fall of the Western Roman Empire destabilised Roman Britain; their most famous victim was Saint Patrick. The raids were discontinued after the British kingdoms consolidated, although an internal trade continued in enslaved prisoners of war and debt slaves caused by famine.
When the Vikings established early Scandinavian Dublin in 841, they began a slave market that would come to sell slaves captured both in Ireland and other countries as distant as Spain, as well as sending Irish slaves as far away as Iceland, where Gaels formed 40% of the population, and Anatolia. Irish slaves in Iceland launched Europe's largest slave rebellion since the end of the Roman Empire in 875, when Hjörleifr Hróðmarsson's slaves killed him and fled to Vestmannaeyjar. Almost all recorded slave raids in this period took place in Leinster and southeast Ulster; while there was almost certainly similar activity in the south and west, only one raid from the Hebrides on the Aran Islands is recorded.
Slavery became more widespread in Ireland throughout the 11th century, as Dublin became the biggest slave market in Western Europe. Its main sources of supply were the Irish hinterland, Wales and Scotland. The Irish slave trade began to decline after William the Conqueror consolidated control of the English and Welsh coasts around 1080, and was dealt a severe blow when the Kingdom of England, one of its biggest markets, banned slavery in its territory in 1102. The continued existence of the trade was used as one justification for the Norman conquest of Ireland after 1169, after which the Hiberno-Normans replaced slavery with feudalism.