The Vikings began raiding Ireland in the 790's AD. Their superior ships, tactics and weapons allowed them to command the seas and the river systems of Ireland. When the Vikings came to Ireland, there were no towns or major settlements. The Vikings created bases that later became familiar Irish towns: Limerick, Cork, Waterford, Wexford and Dublin (orange squares).

By about 840 AD almost all of the monasteries in Ireland were sacked at least once, most several times by the Vikings. Because Ireland did not have a system of towns and what we think of today as civil government, the monasteries of Ireland substituted for civil government. They were often staffed by the elites of society and by strength of family and clan ties were centers of power as well as learning and culture. Some would say that the Irish received the benefits of Roman culture and civil administration via the Irish church without suffering a defeat at the hands of the Romans on the battlefield as most other peoples did. It would be hard to underestimate the role Irish monks and monasteries played in the transmission of Latin and Greek literature and Roman Catholic religion to a post-Roman world.

The historical records speak of mixed offspring of Vikings in Ireland, but just how much genetic mixing took place can't really be determined from the ancient sources. Beginning in the late 830's to late 840's the beginnings of a Norse kingdom took shape in and around Dublin. In 851AD Danish Vikings challenged the Norse Vikings in Ireland for supremacy and the Danes defeated the Norwegians at Carlingord Lough in 851. The Danes did not enjoy their victory long. In 852 Olaf the White sailed with a large feet from Norway to Ireland and set himself up as the Norse King of Dublin and arranged marriage alliances with Irish noble families.

The Norse Kingdom of Dublin was centered around the bases the Norse used to control the river valleys. They did not control more than a small portion of territory, but they did have control of the transportation routes for trade. See the orange-ish shaded areas around the bases of Cork, Limerick, Waterford, Wexford and Dublin. These areas might be places to look for Norwegian genetic contributions to the Irish population.

The Irish were probably spared more waves of Norse immigration or contesting Vikings for the next fifty years when Iceland was discovered in 870 AD. Unlike Ireland, there were no natives to displace in Iceland, so immigrations from Norway to Iceland seemed in many ways more attractive than Ireland. With the Norse in Norway busy settling Iceland, the Norse in the Dublin kingdom were pretty much on their own. The Norse were displaced by the Irish from Dublin for 15 years beginning in 902. In 917 a new Norse fleet took control of the ports of the former Norse Kingdom of Dublin.

The Irish defeated the Norse of Dublin at the battle of Tara in 980, and this effectively ended the Norse dominance in Ireland.