Olaf Guthricsson was the king
and of Dublin
. His father (Guthric obviously), the king of York was exiled in 927, and after this , Olaf ascended to the throne of York. In 939 he became king of Dublin, and having reinforced his position, he became a significant member of the coalition that was defeated in the Battle of Brunanburh
. At that point he was ejected from York
, and went to reside in Dublin
In 939, king Athelstan died, and Olaf could return to England safely. By 940 he had suceeded in regaining control of York, and also controlled part of Mercia as far south as Watling Street. In 942, Olaf died, and his conquests disintergrated, and king Edmund was able to temporarily restore English control of the north.