Ivarr had already been active in Ireland before he arrived in England in 865. After spending the winter in East Anglia, Ivarr and his brothers marched north towards York. The city was, at the time, the capital of the Anglian kingdom of Northumbria, the people of which were engaged in a bitter civil war between King Osbert and his rival Aelle. The Scandinavians actually found the city undefended, and entered it on 1 November 866.
With no one to oppose them, the Scandinavians began to build up the defences of York, to make it theirs. On 21 March 867, the warring Northumbrians put aside their differences and attacked the city. The attack failed, and both Osbert and Aelle were killed. Northumbria, once one of the greatest kingdoms of England, was now a Viking possession.
The victorious Scandinavians did not stop there, however. In 865, the East Anglians had meekly given in to them, handing over food, horses, and winter quarters. In 870, the Scandinavians returned, to kill King Eadmund and take the kingdom for themselves.

When Ivar died in 873, he was called `king of the Northmen of all Ireland and Britain'. Olaf is not heard of after that date and may have also died or retired. We do not know who took over the kingship of Dublin - perhaps Ivarr's unknown son? - but he did not last long: dissent between the different Viking clans gave the Irish a chance to regain Dublin, in 902.