Reasonable
The most likely scenario for an intervention is this;
The Irish Army was to cross the border at Newry and Derry (both heavily Cathloic areas) Derry being under local control at the time, where they would get local support and to allow a controlled evacuation of the Catholic community; and to open up escape routes into the Republic for refugees fleeing the north from Belfast. The Army would then withdraw back across the border when the refugees had crossed.
Bear in mind Armagh & Fermanagh are majority Catholic areas.
This scenario is frightening possible and was seriously considered by the government at the time. Armgeddon was a scenario if the British attacked militarily the refugee routes.
The outcomes are mostly worse than OTL. I find a massive military response from the British unlikely; mainly because there not aggressive idiots, there getting a large part of the unrest been resolved right now, forcing a withdrawal is easy by threats; since the Irish were planning on withdrawing anyway.
The main problem is that the Catholics will likely rise up, badly armed and ill prepared but there desperate and here is the Irish army riding to their rescue!
The Unionists will panic and mobilise. lots of clashes with Irish troops and wholesale burings of catholic neighborhoods, mass executions of "traitors". They'll have to the Irish have invaded and the Brits are obviously selling them out.
Basically the Troubles on drugs with ethnic cleansing and probably state support thrown in the mix. Irish economy suffers and so does their international reputation. The British reputation also suffers when the pictures get out of fleeing refugees and the Loyalist reactions.
Nobody wins.
