Who will win this final duel?

Allah, as in God? Uhm...well. IRA was real*, God...isn't.

If you mean more along the lines of Al Qaeda/Taliban/Hezbollah/ISIS, I'd be inclined to say them over the IRA. I mean the IRA have been around for a while, but they weren't that good in their aims (e.g. Northern Ireland is still British and largely Protestant), whereas these middle Eastern groups have been fermenting for a lot less, but ISIS owning a fucking caliphate.

And if we're going by brutality scales here, ISIS wins by a mile. IRA has their kneecappings, car bombings and occasional taken-outside-and-shot-and-never-disclosing-the-location routine, whereas as ISIS...well, fuck, just a whole higher level of terrorism and brutality.