It's not really considering the three airports in Northern Ireland add up to about 7 million.
It would be interesting to see a map of that, I would bet that the majority of those passengers are from the South East of Northern Ireland.
They are probably basically closer to Dublin airport anyway. I can't see too many from Derry or Ballycastle travelling away down there for a flight to London now can I?
I am travelling out of there for the very first time this summer with United. I priced the flights from Belfast and it was an extra £500.
As it had US pre-clearance we just decided to have a compromise, I haven't been to Dublin in about 10 years so it will be interesting.