It seems rather odd to me that there is such an important distinction that is classified through religious means. When writing, I always refer to groups of different opinions in the troubles as either Loyalists or Nationalists.
It seems there is a lot of Protestants that are Loyalists, and most Nationalists being Catholics. Why is there such a trend among the two religions?
Someone may come along and discuss the religious breakdown of sides in the Troubles in more detail for you, but in the meantime, you might be interested in a previous answer I gave about the history of ethnic and religious tensions in Ireland: https://www.reddit.com/r/AskHistorians/comments/edg4hi/how_british_were_the_irish_considered/?st=k5d1iphz&sh=532dc67f
The key takeaway is that Catholics were the majority but were legally discriminated against for a significant period under Anglican rule. Again, I go into more detail in the above linked answer.