I have a British Army surplus caubeen. Unfortunately, it's a bit small. I can ram it on my head, but it's annoying. It seems the squaddies get them over large from the stores and then shrink them, so if you get a used one they are always smaller than the marked size, and that's the problem. A NATO size 57 should fit me no problem, but it's too tight! Just a warning to others.

As for balmorals being Irish headwear, no. The Canadian Army wear 'caubeens' with a tourie on top, but it's stretching the meaning of the word caubeen too far.

ETA: Pipe bands wear them with hackles. The different colour hackles did represent different British regiments, but for obvious reasons none of them wore an Irish Tricolour hackle, and I have seen those about. I would even wear a green one, because the green has other meanings also not exactly related to the British Army. Just don't for heaven's sake wear a regimental badge!