I suspect that a dearth of readily portable wealth had some thing to do with their lack of plundering, once the monasteries had been plundered, the only other major source of transportable wealth in Ireland was cattle and cattle can be a bit difficult to take on an extended voyage in small boats.
I was reading the other day that for about 300 years the Frisian people (the original Angels, in Anglo-Saxon) were no longer able to live on their islands in the North Sea and the Baltic so they set up settlements in Scandinavia and on the European mainland ~ much to everyone's distress.
One strange group eh?
Anyway, their big business was the slave trade, so it's possible the Scandinavians themselves weren't really doing the slave raids.