[wideawake]

I'm going to guess that the Catholics are more likely to attend parochial schools and the Protestants are more likely to attend state schools.

That would account for the difference.

[Murtyo]

I think all education is funded by the state in Northern Ireland, but the Catholics go to Church run schools, while Protestants tent to go to State run schools. Maybe that accounts for it. They are pretty wide disparities.