I have always wondered why we bother. Genuine question, if given a referendum what would they do?
At the moment they would vote to stay in the UK. But the gap in the religious background is narrowing. At the last census 48% were protestant or brought up protestant, 45% catholic, compared to 53% and 44% 10 years earlier. Perhaps in 10-20 years time the catholics will be in the majority and demands for a referendum will increase?
But even if there eventually becomes a majority in favour of unification there would be huge and probably bloody opposition from many protestants
http://www.bbc.co.uk/news/uk-northern-ireland-20673534