The correct answer is - religion.
The division of the island of Ireland into two separate units became due to the religious conflict between the Catholics and the Protestants. The religious intolerance had been growing over the years, and people started hating each other and attacking each other just because they were followers of a different branch of Christianity.
Eventually, the intolerance led to a bloody conflict, and the island was split into the Republic of Ireland, and Northern Ireland which became part of the United Kingdom. Until relatively recently, there was a terrorist organization in Northern Ireland, by the name of IRA, that was very destructive, but luckily it was dismembered and destroyed.