One quick refresh later to make sure of some basic facts … I need to bone up on Irish history as it is a bit complex (history was never my favourite subject anyway) but the Government of Ireland act 1920 was the act which partitioned Ireland into Northern Ireland and Southern Ireland (intended to be self governing regions within the British Empire, not independent from the UK but not part of it either).
The Anglo-Irish treaty then set up the Irish Free State (still with two regions?) which was supposed to be self-governing, but again as a dominion within the Empire so not independent.
It is splitting hairs a little I know but NI had to actively exercise its opt out following ratification of the treaty which it did the following day so I think there was about 24 hours where Ireland was “unified” as the Irish Free State and (technically) it was the conscious choice of NI to re-join the Union. Not surprising as the northern counties of Ireland had Unionist majorities (and wasn’t that what the Civil War and the partition all been about anyway?)
Fast forward to the troubles (and I totally agree that the violence was on both sides) and you still have a situation where there is effectively civil war in the North because there remains a sizeable minority of Catholics there who want unification with the (now-independent) South and but a (small) majority of Unionists who are loyal to the crown.
The GFA was a brilliant bit of political sleight-of-hand, but it is predicated on both the UK and Ireland being in the EU. It lead to a situation where there was no practical difference or advantage to be gained by the Nationalists in re-unifying Ireland. They could move and trade freely across the border with no formalities grace à the single market while the North stayed part of the UK which kept the Unionists happy. The fact that the EU existed and both Ireland and the UK were members allowed this to happen and it is one of the significant goods to have come out of it (even one argues this was something of a happy accident).
Brexit pulls the rug from under all this which is why it (GFA and Irish border) has become the spanner in the machine. I don’t think this was given enough creedance at the time - although there is that worryng statistic floating around that something just over 70% of Tory voters would want Brexit to go ahead even if the Union were to break up as a result.
This is the sort of thing that I blame the Leave campaign for - prior to the referendum there might have been some low level grumbling about the EU, interestingly polls from the period show slightly more people in favour of leaving, but the 2016 referendum turned this into a palpable hatred which we still have today (and is why we can’t just cancel Brexit without dealing with the backlash).