About 840, one group under Thorgisl arrived and took possession of the cathedral at Clonmacnoise, which he is said to have turned into his personal palace and used the high altar variously as a throne and as a seiðhjállr for his queen Ota. Ultimately Thorgisl conquered all of Leth Chuinn, the northern half of Ireland, which he ruled from Dublin. Many Viking settlements were established during this period, including Wexford (Veisafjǫrðr or Víksfjǫrðr), Waterford, all of the northern third of Kerry, Skellig, Heystone, Bolus Head, Smerwick, Limerick, and Dublin, and three major Viking kingdoms, at Dublin, Limerick, and Waterford.

Viking Dublin was founded in 841; the city derives its English name from the Norse Dyflin, a phonetic rendering of the Irish Duiblinn ("Black Pool," referring to a dark tidal pool where the River Poddle entered the Liffey). For a couple of centuries this coexisted with a Gaelic settlement further upstream, Áth Cliath ("ford of hurdles"), whose name is still the Irish name of the city.

The Northmen ruled Dublin for almost three centuries, despite various battles that are claimed to have ended their power: in 902 they were expelled, but they returned 917; they were defeated in 980 at the Battle of Tara and again in 1014 at the battle of Clontarf by the Irish High King Brian Boru. (Brian himself died in the battle.) Eventually many instead turned their attention to England and Scotland, eventually taking power in England when Canute the Great of Denmark was installed as king in 1015. The Thingmote, the mound 40 feet high and 240 feet in circumference where they held thing, remained beside Dublin Castle until 1685.