The situation with Ireland (the entire country) is similar to that of India and Pakistan. The root causes are the attempts by the British to divide a country geographically to suit religious factions.
India was partitioned (into India and East/West Pakistan) to accommodate Muslims and the Rest. As has been said, Ireland was divided to accommodate Catholics and Protestants. As with many religions, opposing factions could not get on with each other and so (the British believed) had to be separated.
The problem with this strategy is that, unsurprisingly, not all of �Religion A� wanted to live in �Area A� and they were reluctant to move. So we now have the situation in both these parts of the world where not only are opposing religious factions still living together, but the forced partition of their country (and their subsequent �displacement�) causes even more resentment and strife.
The real cause, of course, (certainly in these two examples) is Britain�s belief that it should and could occupy these lands and solve all their problems by such strategies, backed up by military intervention.
History has shown that this cannot be done, but nobody seems to learn.