Because, Ireland is an island and was invaded by Britain and colonised. Collins sent to negotiate due to DeValera's weakness, agreed to hand over 6 Irish counties to Britain. A lot of people in those 6 counties disagreed with this.
In Northern Ireland (the 6 counties), there was a class and religious divide, based on the planted Lords (protestants) and the native Irish (Catholics).
Catholics were mistreated appallingly over the decades since that divide and they rebelled.
The IRA was originally Ireland's legitimate army. However, after the divide, when the 6 counties came under British rule, and hence British police, the IRA became the name for a breakaway paramilitary movement.
Thus comments several decades of bombings in NI and Britain (and Bloody Sunday in the South and at least one bomb that I can recall in Dublin in the 70's).
It became tit for tat guerilla warfare.
What preceded this was 800 years of occupation of Ireland by British.
There was a border which was patrolled by British Army Soldiers in the 70's 80's and early 90's (not sure whether it was there in the 60's).
Eventually, after years of negotiations, they both decided to down arms. Tanks were removed from Belfast and the IRA cease fire.
If you want to know what Bloody Sunday was, Bono sings about it, google it, but it was basically where a british tank went into Ireland's national stadium and opened fire on civilian spectators merely watching a Gaelic football match.
That's why Ireland is different. Tensions were and probably still are high. Peace is hung by a silk thread. All over 6 little counties in Ireland. But Britain would not relinquish control.
And there you have it. The NI debacle in a nutshell. Probably loads I haven't included, but that's the basics of it as I understand them to be.
Then you had things like the Guildford 4? Remember them? Years spent in prison as innocents.
Lots of British army atrocities. Lots of atrocities by the IRA.
America got involved. Not sure that the UN ever did to be honest. Happy for someone to correct me on that.