|A sign of Viking presence in Ireland:|
a Viking ship built in Dublin c. 1042
There were, in fact, two separate periods of Viking incursion, separated by less than a single generation. The first was from 795 until 902, when (according to the Annals of Ulster, mentioned here) "The heathens were driven from Ireland." Those heathens (descendants and followers of Ivar the Boneless) seemed to hang about the Irish Sea, hassling Northumbria and Strathclyde. They returned to the mainland in 914, taking over Dublin.
Ireland was a good place from which to stage incursions into northern England. It was this clan of Ivar's that produced King Olaf III Guthfrithsson, who succeeded his father to become King of York and was driven out by King Edmund in 942.
Although typical Viking raids tended to plunder monasteries and towns and then depart, Ireland was good land for settlements. Viking and Irish intermarried, and produced a group now called "Norse-Gaels." Between the 12th and 14th centuries, the English referred to the Norse-Gaelic people living in Ireland as Ostmen, "East men," because of their origin in Scandinavia. They were considered ethnically and legally distinct from Irish, and lived in their own communities. The modern Oxmantown, now a suburb of Dublin, derives its name from Ostmentown, where Norse-Gaels lived outside of Dublin. According to a 2006 paper, Norse DNA is still found in the Irish population, especially in the areas of Cork, Limerick, Waterford and Wexford.