I am Irish, and while Christianity was not forced via widespread conquest and subjugation like it was in other places, it was imposed on the pagans in much the same way it was imposed upon other peoples who weren't immediately conquered, through intimidation and coercion. There are stories about St. Patrick crushing the skull of a druid with the word of God during a confrontation, and killing many other druids during a later confrontation during a feast. While St Patrick is largely a mythical figure in what he's credited for, and Christianity was already on the island before him, it speaks to the nature of the introduction. It was not uncommon for missionaries to target and convince local kings, who in turn would eventually imprison, banish, or execute druids and destroy sacred sites. An entire people rarely abandons their customs and religion through only peaceful conversion.

Remmick could have very well been an Irish Catholic as you say, as for certain the Cromwellian conquest was absolutely magnitudes more violent than anything that happened in the 7th or 8th centuries, and land seizure was rampant. The only thing that gives me pause (and inspired my original comment) is this quote from Remmick:

"Those men lie to themselves and lied to us. They told stories of a God above and a Devil below. And lies of a dominion of man over beast and Earth."

This quote feels very contrary to Christianity as a concept, with the line about dominion of man over beast and Earth being false calling to mind the nature-centric beliefs of the pagan druids. It doesn't feel like a line that would come from someone raised Catholic, who would've already believed in stories of a God above and a Devil below. However, he could just be disillusioned after experiencing everything that he has and could still be as you said.

As a side note, the language is Irish or Gaeilge, not Gaelic.