Arguably, the Provisional IRA's most successful operation during the entire course of the Troubles was the Warrenpoint ambush, in which two remote controlled IED's were detonated, and occurred on August 27, 1979. The first detonation hit a truck convoy, the second IED was planted where the PIRA believed the medivac element would respond to care for the soldiers killed or injured in the first detonation. In all, 18 British soldiers were killed, and 6 seriously wounded.
The South Armagh Brigade of the Provisional IRA was responsible for the attack, and the bomb makers are believed to be Brendan Burns and Joe Brennan. Incidentally, Burns was later killed when his own bomb prematurely detonated.
The South Armagh Brigade was considered by far the most capable part of the IRA, and controlled much of the ground in South Armagh, making ground transportation extremely hazardous for the British Army garrisoned there. I highly recommend Toby Harnden's "Bandit Country" as the most in depth study of the South Armagh Brigade of the PIRA. The same brigade was also tasked by the PIRA leadership with carrying out most if not all of the 'spectaculars' (to use their terminology) in England. Frequently, the bombs would be constructed near the town of Crossmaglen in South Armagh and be transported to England in semi-truck trailers.
I think this contravenes our 20 year rule. There are plenty of events to discuss pre-1994 but a full answer would probably need to go all the way through to the Good Friday Agreement of 1998.
Edit: That said, this question is probably more guilty of breaching the rules