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.
Horik II
During Horik's reign, Vikings raided England in 855 and 860 with varying success. The Great Heathen Army arrived to East Anglia in 865 and conquered York in Northumbria two years later, starting the Viking conquest and colonization of much of northeast England. While they are expressly called Danes in the Anglo-Saxon Chronicle, the army appears to have been formed by Vikings operating in Francia and Frisia. Whether the royal power in Denmark had part in the planning and organization is unknown. Later sagas assert that this was the case, depicting the leaders of the invasion as sons of the Danish king Ragnar Lodbrok. However, no such king is known in the contemporary sources. The invasion leaders Halfdan and Bagsecg were known as "kings" and may or may not have been related to the kings in the Danish homeland.