Why do you say that? The IRA has no tangible support in the ROI. And I don't believe the Irish citizenry will allow SF to gain too much, if any, of a substantial foothold, in the gov't.
Our PR/multiparty system has the flaw that a fringe party could hold the balance of power, if you check the last Irish General Election results - you'll see the numbers were right for a FF/SFIRA coalition.
Now, you are correct that the majority won't stand for it, which may then mean a second civil war.
In the very least, I predict political unrest over here, if not another Irish Civil War, then perhaps the general unrest of the 1930's - the IRA/Blueshirt/FF dispute.