The first Viking raids on Northumbria, including the church at Lindisfarne, had taken place in the 790s. Shortly after, Iona and Skye suffered a series of raids, culminating in the slaughter of 68 monks of Iona in 806. In 839 a great defeat in which the flower of the Pictish nobility was destroyed was reported. Viking pressure can account for the shift of the Ionan Church eastwards and indirectly explain the emergence of the kingdom of the Scots in a new guise in the early 840s under Kenneth mac Alpin. The Scandinavian threat increased as the 9th century went on. In 870, Vikings took Dumbarton Rock (just west of Glasgow) and the kingdoms within Scotland were threatened by a pincer, from the Danish capital of York and the Viking capital of Dublin. Scandinavian pressure was felt, too, in the north: the earldom of Orkney extended as far south on the mainland as Ross and Cromarty. By the 930s, though, the classic Viking era was nearing its end. The evidence of placenames and of grave goods begins by then to suggest settlers rather than ferocious raiders.
Norwegian adventurers joined Danish Vikings in subjugating much of northern England (the Danelaw) before settling there as farmers and traders and developing great mercantile cities such as York, while gradually extending their settlement into unconquered northern areas such as Cumbria. They also took over the Northern Isles of Scotland (Shetland and the Orkneys), the Hebrides, and much of mainland Scotland as well. In Ireland they played a lusty part in the internecine squabbles of rival Irish clans, and they founded Irelands first trading towns: Dublin, Waterford, Wexford, Wicklow, and Limerick.
Microsoft Encarta Reference Library 2002.