Apples and oranges really. Al-Qaeda is inspired by their Muslim religion. The IRA is inspired not so much by their Catholic religion but a territorial and oppression factor that has more to do with Ireland and the Irish people and Irish land and Irish rights than it has to do with Catholicism. If you took out the Catholic versus Protestant religion factor and replaced it with something else (like Quaker versus Methodist or whatever), the nature of the conflict in northern Ireland would remain pretty much the same. Not so with Al-Qaeda. Al-Qaeda are fighting a fundamentally RELIGIOUS war based on their RELIGION which is Islam (they want to establish Islamic theocracies all around the world as much as possible) Also, there are distinct BRANCHES OF ISLAM which support terrorism. There's no branch of Catholicism that supports terrorism (IRA is not a branch of Catholicism anymore than the Italian Mafia is a branch of Catholicisim -- but with Islam there are branches like the WAHABI (sp?) branch which supports terrorism).
In addition to all that, there are Islamic terrorist groups OTHER THAN Al-Qaeda. So Al-Qaeda is a subset of Islamic terrorists groups. The IRA is not (it's not really a Catholic-oriented group in the first place ... some of its members are probably not even Catholic ... and there are no other terrorist groups that are Catholic anyway).