Fair point. I wasn't aware that quite so many people live in the north. I had imagined that with the shrinkage of the population in the South it might be ~ 4 million for the whole island, probably closer to 6, from what I can see on-line.
You do make a good point though. The APD situation out of UK airports also makes DUB a lot more attractive. It's a fair old whack now, I think. Something like £30 per long haul journey, rising significantly for Business/First.