Sometimes the communists did more than infiltrate. They saw an aggrieved group and actually shepherded them into "organizing for a cause." I was in Ireland in January 1969 when the buzz all over was that there were going to be "protest marches" in the north during the coming summer. These students, bartenders, and young workers did not simultanesously decide to visit Belfast, Dundalk, Derry, etc for their summer holidays. I am not saying that IRA leaders are or were communists, but willing partners when the suggestions to mobilize were made.
The resentment between Catholics and Protestants was real and ageless, but the shooting war was subtly instigated by communist agitators, I am sure.
Actually, the IRA leadership were Marxist to the core... And they trained in Soviet-sponsored "training camps" alongside such groups as Baeder-Meinhof, PLO, PLFP, JRA, Red Brigades, etc...