If you invade Ireland or France, you are invading the EU, which has military capability, and has other allies like the US that also have military capability. Ireland is probably not big/rich/strategic enough to warrant taking on those costs. An invasion doesn't pass a narrow cost-benefit analysis. Throw in the diplomatic costs and it looks even worse.
Edit: I suppose I've stumbled on some kind of economic or game-theory angle here - Militaries exist to impose costs. They tip the balance of potential invaders' cost-benefit analyses. They also impose costs on their owners though. But I guess the idea is that the benefits (avoiding the costs of being invaded) are supposed to be greater than the costs of the defense force.
If you invade Ireland or France, you are invading the EU, which has military capability, and has other allies like the US that also have military capability. Ireland is probably not big/rich/strategic enough to warrant taking on those costs. An invasion doesn't pass a narrow cost-benefit analysis. Throw in the diplomatic costs and it looks even worse.
Edit: I suppose I've stumbled on some kind of economic or game-theory angle here - Militaries exist to impose costs. They tip the balance of potential invaders' cost-benefit analyses. They also impose costs on their owners though. But I guess the idea is that the benefits (avoiding the costs of being invaded) are supposed to be greater than the costs of the defense force.