I've never understood why forcing the majority of population of a province into a country they don't want makes sense.
It usually doesn't.
As Patrick rightly points out, of course, the borders of Northern Ireland are a classic gerrymander - the maximum territory a pro-British government could hold in 1921 while following a semblance of democratic procedure, and not actually the territory that wanted to be split off.
Whether a United Ireland today would work out all that well is quite a different question... the unusual amount of ties that were never ruptured during the division is a good sign, but in general geographical cleavages are easier to create than to undo.