Well a nice reasoned approach to Islam like that would be so nice
The crucial difference is that Catholic organisations universally, unequivocally and without the use of the word "but" condemned violence. They didn't claim they were forced to do it by the British Army, not least because, as we're doing historical accuracy, the original reason for the British Army to be posted to Belfast was to protect Catholic communities who were being attacked by the UDA; it got a lot more complicated after Bloody Sunday, obviously. They didn't claim that the violence was justified by cause of nationalism or of republicanism, they didn't propose that the British government hand over the six counties to Ireland and then stand idly by while the inevitable bloodbath ensued, and they didn't say that their were forced into it by hussies on the mainland using contraception. They just condemned violence, without equivocation.
There was, for practical purposes, no fifth column, which was why there was never a serious existential threat to the UK. They lacked the support, even in the six counties, never mind Ireland, and there was extremely limited supported within the Irish community on the mainland, where extremely limited meant "undetectable". There was probably more support from wankers on the hard left who thought the IRA were a jolly fun bunch of ruffians than in your local Irish centre. That was why the IRA found it very difficult to operate on the mainland.
They were also asking for a deliverable political agenda and were willing to negotiate on it (as the Good Friday agreement showed), had reasonable paramilitary discipline (as the delivery of post-Good Friday decommissioning shows, along with the generally democratic behaviour of Adams and McGuinness since then) and realised that if they behaved like psychopaths they would lose such little support as they actually had (it was the Omagh bombing that finished even the offshoot IRA organisations: killing pregnant women and children disgusted even their staunchest supporters).
There was, for practical purposes, no flow of British Catholics to Ireland to join in the bloodshed, no fundraising in the UK (the US is a different story, one which tragically it took 9/11 to end) and no revelling in bloodshed.
ISIS do not have a deliverable political objective, nor are they willing or able to negotiate in any case, and their violence presents a genuinely existential threat to the west. There is simply no comparison with what was, in the end, a bunch of student radicals with slightly better weapons, who did not want to end up dead and had a vaguely deliverable political objective.