There were contingency plans to invade Southern Ireland. This was in two parts. Specifically if they allowed Germanicon submarines to use their ports or allowed the Country to be used in any other way by the Germans the Country would have been blockaded at sea and if that failed (and it's extremely hard to see that failing.....) the structurally weakened Country would have been invaded There was also a severe contingency plan to severely punish them should they ever shoot down or harm an allied plane entering or returning from the Atlantic along/through the specified corridor. This corridor into the Atlantic from Northern Ir across Southern Ir was put forward as what you could/might call a fiat-accompli. So far as I'm aware, apart from the fiat-accompli or 'or else....' aspect of the 'agreement', it was never abused by either side