Ok, fuck it, let's do this.
Taking advantage of the vagueness of the original post I'm going to create the only scenario I can see that could possibly lead to this happening, and not involve Ireland winding up invaded and overrun by teatime, and to do it I'm going to throw rationality and sense out the window and get as close as I can without it literally involving ASB (but it will be fecking close).
To start with, you have the Falklands war go a hell of a lot worse for the British at the offset. In this scenario the Exocet's wind up pulling a golden BB hit on every British naval vessel they hit, and firing them even more often, sinking them with all hands. We also in this scenario have the Argentines be much more aggressive in their deployment of the missiles and, for good measure, the Argentine submarine fleet winds up being a much greater and capable threat.
You start having enough British naval vessels go to directly connect to Poseidon's wifi with all hands that the war becomes immensely unpopular and Thatcher starts getting backed into a corner. You then have a Royal die (IIRC Prince Pizza Express in Woking was involved in the war somewhere) so that the British people have to see the Queen grieving on the Telly which is the final nail in the coffin to just walk away from the war and take the loss. Thatcher is consigned to the dustbin of history much earlier and the UK winds up with a different interim PM until the next election which sees Labour win a landslide.
Freshly traumatized from the conflict, the British people develop a very strong isolationist mindset wanting to just get rid of all the little pockets of empire before they have to see more of their children die, so you see the likes of Hong Kong handed over early and anything else that they feel there is a possibility of a fight needed to keep just being thrown to the winds. In this calculation, Westminster takes a look at the ongoing hell scape that is Northern Ireland in 1982 and decides "Fuck it, we agreed Sunningdale, that's going to be it, they can sort themselves out" and does the equivalent of the US leaving Saigon with British officials and troops, leaving behind whatever they can't carry. Britain basically declares Northern Ireland its own state and wishes them good luck.
This, naturally, devolves into an orgy of violence and destruction that makes the rest of the Troubles look like a mild disagreement between under 12's camogie teams and the entire world is left stunned at this. Britain, choosing to refuse to do anything to resolve the matter continues to do the international version of sticking its fingers in its ears and humming loudly so the only other player in the area that could maybe do anything is put under international pressure to go in and bring peace, leading to the scenario where against their will and probably calling up not only the entire FCA but also any retired members who are still in possession of two arms and two legs, the Irish Defense Forces is sent into Northern Ireland.
To say this does not go well would be an understatement as you would have the PUL community, who already feeling pissed off and betrayed by London would throw every bit of anger at the invading Fenians in the sort of persistent guerilla war that even the best equipped militaries do not want to fight. You have the CNR community that, while some may feel relieved, has as its main armed group a terrorist organization that views Dublin as illegitimate and the Defense Forces as much a legitimate target as the British forces. The only difference being that Ireland won't hesitate to call in the Blue Helmets to help out in this matter, which I personally am picturing leading to the "hilarious" situation of Congolese, Cypriot and Lebanese forces in time doing Peace-Keeping in Ireland.
Convoluted? Yes. Relying on so many things to go exactly one way and one way only? Yup. So borderline with ASB that the scenario counts as a Frontier Worker under EU employment laws? Ja. But there you go, the only non-ASB way I could see this happening.