Sod all if one of the members is on the Security Council.
Article 5 involves taking action against a member state that the Security Council itself has placed in the naughty corner. Predictably Russia (PRC wasn't a member until 1971) would have vetoed any attempt to declare it against Ireland. Britain would have vetoed any measure to put them in the same spot and for all the noise that might have been invoked by the plastic paddies in Boston and New York, I doubt the US government would want to fuck around with a key ally who had been, however lamely, militarily attacked. They certainly didn't in 1982 when much more was at stake in terms of their geopolitical strategy. I think Al Haig did enjoy the frequent flyer points though.
Purely for moral suasion, the British government could also have invoked a somewhat more relevant Article 5 in the NATO treaty. Not that it would have required foreign assistance, just nice to put your opponent on the wrong side of the fence.
The hidden cost for Ireland would have been the effect on its pending EEC membership application. Probably not an endearing quality in a potential member state.