Well a quick search says that the Norse-Gaels dominated the Irish Sea until the Norman period and their descendants had some power until the 16th century. Wikipedia says that the legal protections that the Norse descendants enjoyed in Ireland were eventually fazed out in the 13th and 14th centuries as they had been completely assimilated.
Personally I suspect that the majority of the Norse were likely assimilated by the 12th century. It was never widely spoken in Ireland and that country has a history of very quickly assimilating invaders.
I think making Norse last longer would take a fairly major POD to seriously push up the number of speakers in Ireland. Perhaps both England and Ireland are completely divided into Norse kingdoms?