Spoiler: implausibilities
Generally I liked the film but I have a problem in particular with the script's handling of Boyle, the senior IRA man, which I find implausible.
1) He is collaborating with the British Army, so it is unbelievable that he would tell his juniors to stand down in their efforts to capture a British soldier as it would expose him. It's the gangland equivalent of an FBI mole in the mafia leadership telling his foot-soldiers NOT to whack a bent cop when they have a golden opportunity to do so. It's too screamingly obvious.
This said, IRA protocol may indeed have required him to tell his juniors to stick to their orders rather than go after targets of opportunity, meaning that his argument that he's maintaining discipline could be convincing: however, this would need to be spelled out better for the audience, and anyway if it were true that his actions were in keeping with IRA policy then that would in turn remove the plausibility of his juniors perceiving his behavior as collaborationist.
2) There's no explanation about when and how Quinn works out that Boyle is a collaborator. The only clues the script gives us to go on is (i) Quinn's suspicion after Boyle's order for him to stand down and (ii) Boyle's comment to his juniors that he'll be "unable to protect" them if they continue to disobey orders. Presumably, Quinn interpreted that comment as Boyle inadvertently letting on that he had some kind of unusual ability to protect his men from the British, therefore Boyle had to be working for the British. But there is no moment when we the audience are shown this interpretation by Boyle's juniors. The perfect opportunity to show this would be when Boyle's juniors are walking away from meeting him in the pub and one accuses him of lying, although it appeared to me that they were referring to Boyle lying about not being responsible for the bomb in the Protestant pub. Rather, one should have said to the other, "What kind of a commanding officer stops his men from capturing the enemy?" And the reply: "Only a commanding officer who is working for the enemy." Still, even though this would make the chain of events more plausible, it wouldn't solve the implausibility of Boyle's overtly collaborationist behavior in the first place.
3) Once the British soldier is found, Boyle immediately calls his British Army contact to hand him over. But he has no motivation for handing over the British soldier other than to ask the British (who he doesn't entirely trust) to kill Quinn. He would more easily have ordered Quinn executed by the IRA himself, on cooked up charges of collaboration. And why take yet another risk of unmasking himself to his juniors by arranging for the handover of the British soldier? It's implausibly stupid behavior.
4) He arrives with the British to pick up the soldier, which is an open declaration to the Catholic community that he is a traitor. Why would he do this? Not only does it give him an immediate death sentence from the entire IRA and ostracizes him from the entire Catholic community, it is also completely unnecessary - the British could easily have picked up their soldier alone (it's not like they had ever used locals to politely knock on the door rather than smashing it down). Implausible.
5) All of this begs the question - what is the purpose for the plot that the juniors suspect that Boyle is a collaborator? It appears the only value that the suspicion has is that it motivates them to follow him - as it turns out, to the soldier's hiding place. But really, that's like using a sledgehammer to crack a nut. The panicking family could easily have called BOTH Boyle and Quinn to the scene, removing all of this implausible and unnecessary plot-line of Quinn suspecting Boyle.
6) Finally, another implausible plot point is that the British covert operations unit would attempt to murder a British soldier for the sole reason that he witnessed them supplying Protestant terrorists with a bomb destined for the IRA. In the real world, the soldier would be briefed that he had witnessed a classified operation in support of his tour of duty and that as a matter of national security he has to speak about it to no-one or face court martial and jail. Moreover, the murder of the soldier was surely going to be pinned on the IRA, but it came in the form of strangulation; this was totally pointless when there was an IRA gun in the room, especially given that the IRA executed with a bullet to the head, never by way of strangulation. And to cap it all, why does the British covert operations unit intent on murdering a British soldier invite the British Army along for the ride? Even though they are told to keep their distance, it's still an idiotic risk of exposure that covert operations operatives who happen to be premeditated murderers with rat-like cunning simply do not take.
I'd very much welcome comments, especially if someone can persuade me that I'm wrong and all of this is in fact plausible. That will make my second viewing much more enjoyable. By the way, I like the film a lot, which is why I'm nit-picking.