Nothing really they are both murderous bastards, the only difference is the Al Qaeda bomber has the faith to die for his cause, while the IRA bomber planted their bomb ( often funded by Americans, especially between the late 80s-mid 90s ) and then detonated it from a safe place outside the blast zone.
They both target huge crowds, Al Qaeda markets, busy areas, the IRA train stations department stores before big holidays, like Mother's Day and Christmas. The end result is the same, they kill innocent people and do not endear the public to their cause.
