Second Viking age of Ireland

After being expelled from Dublin in 902, the descendants of Ivar, referred to as the Uí Ímair, remained active around the Irish Sea, engaging in activities in Pictland, Strathclyde, Northumbria, and Mann. In 914, a new Viking fleet appeared in Waterford Harbour, followed by the Uí Ímair who reasserted control over Viking activities in Ireland. Ragnall arrived with a fleet in Waterford, while Sitric landed at Cenn Fuait in Leinster. Niall Glúndub, who became the Uí Néill overking in 916, attempted to confront Ragnall in Munster but without decisive engagement. The men of Leinster, led by Augaire mac Ailella, attacked Sitric but were heavily defeated at the Battle of Confey (917), enabling Sitric to re-establish Norse control over Dublin. Ragnall then left for York in 918, where he became king.

From 914 to 922, a more intensive period of Viking settlement in Ireland began, with the Norse establishing major coastal towns including Waterford, Cork, Dublin, Wexford, and Limerick. Archaeological excavations in Dublin and Waterford have unearthed significant Viking heritage, including burial stones known as Rathdown Slabs in South Dublin. The Vikings founded numerous other coastal towns, and over generations, a mixed Irish-Norse ethnic group, the Norse-Gaels, emerged. Despite the Scandinavian elite, genetic studies suggest that most inhabitants were indigenous Irish.

In 919, Niall Glúndub marched on Dublin but was defeated and killed by Sitric at the Battle of Islandbridge. Sitric left for York in 920, succeeded by his kinsman Gofraid in Dublin. Gofraid's raids showed some restraint, suggesting a shift in Norse strategies from mere raiding to establishing a more permanent presence. This shift was evident in Gofraid's campaigns in eastern Ulster from 921 to 927, aimed at creating a Scandinavian kingdom.

Muirchertach mac Néill, son of Niall Glúndub, emerged as a successful general, defeating the Norse and leading campaigns to force other provincial kingdoms into submission. In 941, he captured the king of Munster and led a fleet to the Hebrides. Gofraid, after a brief period in York, returned to Dublin, where he struggled against the Vikings of Limerick. Gofraid's son, Amlaíb, decisively defeated Limerick in 937 and allied with Constantine II of Scotland and Owen I of Strathclyde. Their coalition was defeated by Athelstan at Brunanburh in 937.

In 980, Máel Sechnaill mac Domnaill became Uí Néill overking, defeating Dublin at the Battle of Tara and forcing its submission. Meanwhile, in Munster, the Dál gCais, led by Cennétig mac Lorcáin's sons Mathgamain and Brian Boru, rose to power. Brian defeated the Norse of Limerick in 977 and gained control over Munster.

By 997, Brian Boru and Máel Sechnaill divided Ireland, with Brian controlling the south. After a series of campaigns, Brian claimed kingship over all of Ireland by 1002. He forced the submission of provincial kings and in 1005, declared himself "Emperor of the Irish" at Armagh. His reign saw Ireland's regional kings submit, but in 1012, revolts began. The Battle of Clontarf in 1014 saw Brian's forces victorious but resulted in his death. The period after Brian's death was marked by shifting alliances and continued Norse influence in Ireland, with the Norse-Gaelic presence becoming a significant part of Irish history.