FN FAL would also be accurate. It would technically be even more accurate if it was the semi-auto L1A1 version, as then it could have been "liberated" from the British military, but I'm fairly confident they would have been able to get actual FAL's.
Thor's right, the L1A1 (the semi-automatic version of the FN FAL) and its variants served as the British standard issue service rifle from 1954-1985. Since the beginning the IRA got most if not almost all of their weapons and equipment from the the UK. Mostly stolen or illegally purchased.
They also used a decent amount of the civilian AR-15 model of the M-16 so you could go that way as well. Interesting idea though. Good luck!