it's a mixture of the two.
Ireland, Scotland and Wales always have the ability to produce their best performances against England. It's not a coincidence. Which is why, at times when England on paper and form are a vastly superior team, our Celtic rivals can produce the upset, eg. 1999 Wales, 2000 Scotland, 2001 Ireland etc. etc. None of these upsets occur with the tri nations, do they?
Ireland's recent dominance is partly that, but more to do with the fact that since 2004 Ireland have simply been a better team - in fact, the generation of BOD, ROG, POC etc. has always had the ability to be one of the world's better teams. So even when England are better, like this year's 6N, the Irish had the ability in them to be fired up enough (probably by the prospect of spoiling England's party) enough to completely blow away a good team.