Yet nobody blamed ALL catholics for the IRA attack. No one said, 'he's a catholic he must be a sympathiser'. No one attacked or set fire to catholic churches on the mainland. Strange eh.
The blame was rightly laid at the door of those who carried out the attack. As it should be in this case.
Whilst this is true there are some differences.
Firstly although sides in The Troubles were essentially formed along sectarian lines the driving issue was not religious ideology but political. The IRA were not claiming some religious justification but a political one.
If there had been a viable political solution open to Loyalists then perhaps The Troubles might not have escalated.
ISIS goals, on the other hand, are theologically driven. They don't care about dialog or compromise. They want all infidels to be dead and nothing we can do can change that view.
Whilst ISIS represents the most extreme interpretation of Islam and one that most Muslims thankfully do reject, it is still based on texts that put a division between the believer and the non-believer. The Quran, just like the Bible, creates a moral barrier between the two groups that allows people even at a moderate level to build a view that they are better than a non-believer. It also means that at some point a believer has to decide if they are going to accept their holy text or not when faced with certain situations (e.g. homosexuality, sex outside of marriage).
In most forms of Christianity the moral barriers have been broken because people don't care and the efficacy of social disgrace has diminished. 50 years ago single mothers were still going to homes to have babies in secret but that's been wiped out as people simply don't care.
In the West, those that wish to have a Christian faith are generally free to choose how they mix this with anything else that influences their lives be that secular or not. No one cares.
It's only in smaller faiths or in countries where progressive views are still in the minority that a strict view of Biblical morality holds sway - like Jehovah's Witnesses. Even then they have to use shunning as a social control. Even fewer Christian sects are anywhere close to using violence to try and force their views on people.
This is not the case in Islam. Even many so called moderate Muslims live in families and communities where any divergence from accepted norms results in pretty severe social consequences. Muslim women, even in the West, suffer from a far more patriarchal infrastructure than most non-Muslim women. So many Muslims have a problem trying to find a happy compromise between Western permissiveness and secularism when contrasted with their traditional views, even if considered fairly moderate.
This is why the answer to this has to come from within. Of course it's impossible for an individual Muslim to stop extremist terrorism but that's no reason not to examine what they are really signing up to and rejecting anything that negates tolerance and inclusion. Just like with Christianity, if enough Muslims said they didn't care then change could come.