While the Republic of Ireland generally does not send its armed forces abroad, Irish citizens are allowed to enlist in the British Armed Forces. It might be possible that he served in the British Army in several of its conflicts - Korea, possibly, the Falklands, or one of several conflicts of decolonisation. It seems unlikely that a headteacher would advertise former IRA membership, but if he was in the British Army it's possible he was on the other side of the Troubles; the IRA employed bomb, mortar, machine-gun, and sniper attacks on British forces.