First of all I should make a note and disclaimer that there was so many splits in the IRA that not all of them were purely religiously motivated. So your question is quite valid. At the same time I was not intending to say in my post that the IRA had purely religious roots, but however, that a signficant chunk of the root cause of their existence is rooted in religious discrimination.
For the rest of the answer I am going to present this wikipedia article:
I think you’re going to be surprised by how far back this goes.