We had some casest of forced population transfers in the 1920s, like the turks with the greeks and the armenians after the greco-turkish war or the poles and germans in silesia.
Assuming the british sees this as a definitive solution to prevent dissent like the troubles, and negociated for Ireland to receive all the northern irish catholic population to recognize their independence, and then received the scant irish protestant population on northern ireland, and also used poor british protestant citizens (be scottish, from wales or english) to repopulate northern Ireland, could the british do it? How Northern Ireland would look like today? What would be the historical repercussions?
You can't have forced population transfers without mass killing. 'Cuz people are pretty attached to their homes.
And as others have mentioned, the communities in Ireland were very close and intertwined. Religious persecution on this level would leave lots of protestants in Northern Ireland upset. And of course, there's all the protestants in Southern Ireland to think about. What happens to them if the British are persecuting catholics in the North?
This will also wreck relations between Ireland and the UK. While the republicans won independence for Southern Ireland, there were still plenty of people with an overall positive view of the British. Which would be a factor in the pro-treaty forces winning the Irish Civil War. If this is going on before or during the Irish Civil War, there's a very real danger that the anti-treaty forces win, which is to no-one's best interests. If it is after the Civil War, the British still lose alot of goodwill amongst the Irish population British business and diplomacy suffers accordingly.
And this won't help the British on the international stage. I predict the French and the American reactions will be especially important.
If you look at the legacy of forced migration in Europe, it is clear that the long-lasting traumatization of millions of people has not been worth the blood. Brutalizing real people in order to make populations conform to ethnolinguistic and religious ideals has resulted in whole populations who still suffer from the trauma generations later that also have consequences for the economic output of the populations that were so brutalized.
There are far better and far easier ways to avoid The Troubles of Northern Ireland.
fasquardon