Olaf Guthfrithson (ōˈläf gŭthˈfrĭthˌsən) [key], d. 941, Norse king of Dublin (934–41). His father, Guthfrith, king of Dublin and of York, had been driven out of England by Athelstan in 927. Olaf led (937) his allies, Constantine of Scotland and Owen of Strathclyde, against Athelstan in the battle of Brunanburh and was severely defeated. He returned to Ireland, but after Athelstan's death he invaded (939) York. A treaty between Olaf and King Edmund, successor of Athelstan, gave Olaf control over Northumbria and part of Mercia. His successor, Olaf Sihtricson sĭtˈrĭkˌsən, d. 981, often called Olaf Cuaran, was expelled from Northumbria by Edmund in 944. He returned to rule in Ireland, where he was in 980 defeated at the battle of Tara. He died a penitent at the monastery of Iona.