Roman Catholic is certainly the largest single religion in Northern Ireland. According to Wikipedia, just over 80% of the population are Christian, with roughly half Roman Catholic, and Presbyterian in second place. Here in the Republic of Ireland, about 80% are Roman Catholic.
What I find sad about Northern Ireland in the division between the groups. I was in Belfast about a year ago, and found all the people really friendly. I didn't care what religion or allegiance (unionist or nationalist) the people held. While I was there, I did the tourist bus tour, and got to see the so-called "peace lines" - 18 foot barriers which separate the two halves of the community. Building walls really shouldn't be necessary in the 21st Century.