Two large Norse fleets of sixty ships each appeared on the Boyne and Liffey rivers in 837 AD. Led by Viking chief Turgesius (Thorkell), who was returning to Ireland for the first time since his first successful raid in 820 AD, they were a formidable force. The Irish Annals report that there was no significant unified resistance from the Irish, which allowed the Vikings to establish a longphort (naval base) in Annagassan, Co. Louth and another in Dubh Linn (Dublin), at the boggy tidal pool in the River Poddle where Dublin Castle now stands.

The Vikings wintered in Dubh Linn for the first time in 841 AD, and in 842, reinforcements arrived. This is also the first year Irish-Norse alliances are mentioned in the Annals of Ulster. Also recorded are the Viking raids on Bangor, Armagh and the churches on Lough Erne during this period. In 839 AD, they plundered churches in the north of Ireland from their longphort at Lough Neagh and again in 852 from Dubh Linn. After sacking Armagh three times, they were in turn sacked. When, in 845 AD, they raided the Clonmacnoise monastery, Tuirgeis is said to have enthroned his wife, Otta on the high alter, from which she delivered oracles. He then captured the settlement of Armagh and appointed himself king. Later that year Tuirgeis was murdered. Some say Irish King Malachy of Meath drowned him in Loch Owel, while others say the king's daughter lured him into a trap and he was ambushed by Irish warriors disguised as maidens.

The Norwegian Amláib (Olaf the White) arrived in Dubh Linn in 853 AD and ruled together with Ímar the Dane (Norse: Ívarr inn beinlausi "Ivar the Boneless"). The Annals of Ulster report that Amláib and Ímar raided northern Britain in 871 AD and returned with two hundred ships and an enormous cargo of captives and animals. Ímar died in 873 AD as self-proclaimed "king of all the foreigners in Erin." According to the Scottish Chronicle preserved in the Poppleton Manuscript, Amláib returned to Norway after Ímar's death to help his father Gofraid (Guthfrith) fight the Norwegians, leaving the kingship of Dubh Linn unstable for the duration of the century.

In 902 AD, the Dubh Linn longphort was attacked and the Viking inhabitants fled, abandoning most of their ships and barely escaping with their lives. In 917 AD, they returned with reinforcements and established a successful trading emporium, which appears to have been the only Viking centre of commerce in Ireland during the latter half of the ninth century.

Dubh Linn rapidly developed into a thriving town with the largest and most politically active of all the Scandinavian-controlled Viking Irish hinterlands called the Dyflinarskiri, which included modern Co. Dublin and parts of Wicklow reaching to Arklow, with Leixlip (the salmon leap) marking its western boundary. Though Viking Dubh Linn maintained political independence through contact with Scandinavia, the material finds of this second and more intensive phase of settlement indicate that its people were becoming assimilated into Irish culture.