Now that I don't have much to do, I think I'll dust off the old speculation cap.
As others have said, an attack by the IRA on Hogwarts will probably cause an upsurge in anti muggle/muggleborn sentiment, but there are other factors to consider;
Hogwarts is surrounded by hundreds of dementors. At some point in the post attack blame fest, someone is going to bring up the point that, even with the backing of magical people, there's no way an entire strike force of muggles could make it to the castle; much less, make it undetected. So people will realize that the dementors (or their handlers) were convinced to let them past. I don't think Fudge is involved. Even if he were convinced to give clearance without knowing about the IRA, it would be simple to trace the attack back to Lucius, or whoever talked him into it. Because of the dementor angle, there's no way that Fudge will be knowingly involved, as the attack is certain to cost him (or at least, seriously endanger) his position; particularly coming on the heels of Mellisa's death.
Bottom line is, whoever is behind this, at the very least, doesn't care if Fudge is sacked.
As to the attack itself, I don't think that any of the wards are geared towards protecting against modern muggle military ordinance. While it would probably be fairly simple to develop wards that do so, there won't be any at present. If the IRA try to sneak into the 'abandoned' castle by say, using explosives to blow a hole in the perimeter wall; I think that might disrupt the wards in that section. Which brings me to a possibility I've been wondering about;
Assume the theory of Voldemort somehow being aware of the original timeline is correct. What would be the first thing he would do, knowing that part of this Harry is the Harry Potter who knows all his secrets and who fought and killed him in the original timeline? Retrieve his horcruxes from their original hiding places of course. Except, he finds that the ring, locket and the cup are already gone. Potter is faster than he thought. What did he do in the original timeline when this happened?
Invade Hogwarts, massacre the populace and steal the sorting hat horcrux. But why stop there? He can catch the only two wizards who pose a threat to him off guard and kill them while he's at it.
The IRA unwittingly breach the wards, the dementors, who he has secretly converted to his cause (thus allowing the IRA past) swarm in through the breach. Azkaban is currently being controlled by someone pro Voldemort, so Voldemort has access to Bellatrix and all his imprisoned followers, as well as the ones who avoided imprisonment and Barty Crouch Junior. Voldemort has used some other method to regain his body and powers and used occulemency to prevent Harry from realizing this via their link. He even knows about the twin wands, so he knows to use a different wand.
None of this will probably happen, but it's fun to speculate.