Early Scandinavian Dublin
In the year 795 Vikings (probably of Norwegian origin) raided islands off the coast of Ireland for the first time. This was the beginning of a new phase of Irish history, which saw many native communities – particularly ecclesiastical ones – relocate themselves on the continent, or further afield in places like Iceland and the Faroe Islands, to escape the pagan marauders. For about two decades the invaders confined their activities to coastal settlements; raiding parties were generally small and there is no evidence that any of them wintered in Ireland during this early phase of "hit-and-run" activity. Typically the Vikings would arrive at a settlement without warning, plunder what goods and people they could – the people were usually sold as slaves, though notable personages were often held for ransom – before retreating to their Scandinavian or British bases. This period lasted from 795 until 813, after which there occurred a hiatus of eight years.
It is now thought that these early raids were launched directly from southwest Norway, and that during the period of calm (814–820) the Norwegian Vikings were occupied in northern Britain, laying the foundations of a new kingdom referred to in Irish sources as Laithlind (later Lochlainn). Laithlind was once thought to be in Norway but it is now identified with Viking settlements in the British Isles, especially those in Scotland and the Isle of Man.
Ólchobar mac Cináeda
Late in 848 Ólchobar is said to have set up a camp from which the Vikings at Cork were blockaded. The result of this siege is nowhere recorded. This is the first mention of the Vikings at Cork. They are not heard of again until 865. It has been suggested that these several campaigns against Vikings were a coordinated effort by the chief Irish kings.