Amlaíb mac Sitric (c. 927 – 980; Old Norse: Óláfr Sigtryggsson), commonly called Amlaíb Cuarán, in Old Norse: Óláfr kváran, was a 10th-century Norse-Gael who was King of Northumbria and Dublin. His byname, cuarán, is usually translated as "sandal". His name appears in a variety of anglicized forms, including Olaf Cuaran and Olaf Sihtricson, particularly in relation to his short-lived rule in York. He was the last of the Uí Ímair to play a major part in the politics of the British Isles.
Amlaíb was twice, perhaps three times, ruler of Northumbria and twice ruler of Dublin and its dependencies. His reign over these territories spanned some forty years. He was a renowned warrior and a ruthless pillager of churches, but ended his days in retirement at Iona Abbey. Born when the Uí Ímair ruled over large areas of the British Isles, by his death the kingdom of Dublin was a minor power in Irish politics. At the same time, Dublin became a major centre of trade in Atlantic Europe and mastery over the city and its wealth became the supreme prize for ambitious Irish kings.
In death Amlaíb was the prototype for the Middle English romance character Havelok the Dane. In life he was a patron of Irish poets and Scandinavian skalds who wrote verses praising their paymaster. Amlaíb was married at least twice, and had many children who married into Irish and Scandinavian royal families. His descendants were kings in the Isle of Man and the Hebrides until the 13th century.
The earliest records of attacks by Vikings in Britain or Ireland are at the end of the eighth century. The monastery on Lindisfarne, in the kingdom of Northumbria, was sacked on 8 June 793, and the monastery of Iona in the kingdom of the Picts was attacked in 795 and 802. In Ireland Rathlin Island, off the north-east coast, was the target in 795, and so too was St Patrick's Island on the east coast in 798. Portland in the kingdom of Wessex in south-west Britain was attacked during the reign of King Beorhtric of Wessex (ruled from 786 to 802).
These raids continued in a sporadic fashion throughout the first quarter of the ninth century. During the second quarter of the century the frequency and size of raids increased and the first permanent Viking settlements (called longphorts in Ireland) appeared.