You wrote:
“OK so imagine the scenario where Northern Ireland wakes up one morning and finds itself a part of Eire.”
Impossible. It will take years for the handing over of the territory. It would be much like what happened between the UK and China over Hong Kong. Years of talks, an agreement well ahead of time, plenty of time to sort out what everyone would do, etc.
“Its Garda not the PSNI and the Irish Army and not the British who are on the ground.”
Okay.
“What do think is going to happen ?”
Those Unionists not willing to stay will pick up and move long before the handing over. Most unionist hotheads with murder on their mind will also decide to leave because they know they will have backing from no nation or major power on earth to conduct any major terrorist campaigns. Those few diehards with a death wish will stay and soon be moped up since the official power of the state - both the UK and Ireland - will be against them with no hope of victory or even long term survival as terrorists in Ireland. There would also be an economic downturn and mass migration from other countries. A certainly more diverse and perhaps economically more powerful Ireland would ultimately result. And after a few years there would be no terrorist violence to speak of. In other words, Ireland would be better off in some ways than it has been in centuries: united and free.
What a pity that people don't work that way.
Ireland after the immediate setting up of the Irish Free State was anything but a happy and economically secure place. Admittedly there was a major worldwide economic downturn at the time but Ireland certainly suffered more than most.
“Those Unionists not willing to stay will pick up and move long before the handing over. Most unionist hotheads with murder on their mind will also decide to leave because they know they will have backing from no nation or major power on earth to conduct any major terrorist campaigns.”
Which is why it will never happen, they will never leave. They’ve lived through centuries of conflict and will no sooner leave than the people on the Republican side left when NI was formed. No state is going to have the political will to force them out either.