Both IRA campaigns of bombing and assassinations in Northern Ireland and American campaigns of bombing and assassinations against abortionists were motivated by a strongly felt Catholic identity, and a sense of grievance that that identity was not properly recognized as the correct organizing principle of society.

Both could, coherently, be categorized as “Catholic terrorism”. That in America they generally aren’t, and why they aren’t, can be an interesting dynamic to explore in its own right.

That said, it would be damn weird and not very useful to understand the two campaigns as facets of the same single thing.