That's an inaccurate and tired comparison. Whilst it's true that most (but not all) Provisional IRA members were Catholic, Catholicism was never the cause for which the IRA said they were fighting for, rather it was for Irish Nationalism which, if you trace it back to it's origin, was effectively started by Protestants, and men of that sect such as Theobold Wolfe Tone are revered in Nationalist, Catholic circles. The sectarianism of the conflict was a by-product, not a cause, in later years throughout the troubles.
Had the IRA been waging their campaign for the 6 Counties to be a Papal state, then the term 'Catholic Terrorist' (assuming one accepts organisations engaged in that kind are terrorists), would be entirely appropriate. But the perpetrators of the Paris attack (assuming it is ISIS) have openly stated that their end goal is to have the entire world governed by Sharia law under Islamic rule and, as such, labelling them Islamic Terrorists is factually correct. To ignore the fact that their interpretation of their faith is at the root of the trouble, is
IMO, dooming ones self to fail against them. Does that mean all muslims are terrorists? Of course not, that really does go without saying.