It is silly to conflate separatist movements and Christian groups from Europe with Islamic terror. Those were internally generated problems involving natives. No society is perfect, and you do your best to deal with those internal issues.
Those groups also tended to have clear political goals and generally used much more discretion in their choices of targets.
On the other hand, you've got Islam which is:
1.) Alien to Europe and alien in a much more significant way than Christianity was because it is not only an outsider ideology, it is an outsider ideology coming in on the backs of a huge number of actual outsiders. Christianity came in and was adopted by Europeans, big difference.
2.) Deliberately targeting the softest targets, going after defenseless, unsuspecting civilians and in many cases even women and children. Inflicting gruesome torture on victims like removing eyes and genitalia of victims in the Bataclan theater. Slitting throats of people on London bridge yesterday. I'm no expert on the Basque stuff but my understanding is that the IRA typically would target English police & military, or politicians, stuff like that. Not saying that's right, but they had a clear political agenda and weren't trying to take over England or deliberately kill women and children, though I do understand that sometimes their target choices were awful.
3.) Other than conquest of Europe and the world, it's hard to argue that the Islamic terrorists have a clear political agenda. Many people try to make it seem equivalent to the IRA or something by saying "oh they're just reacting to interventions in the middle east" but his doesn't pass the sniff test because of how much violence they do in countries like Sweden, how much internal violence directed at people like Egyptian Christians and Hindus in India, etc. they are known for... and the fact that our meddling in their nations is a REACTION to them being like that, not the cause of it. Again... their agenda is conquest, not redress of grievance. I expect Dublin to have a major Islamic terror attack in the next couple of years. What will be the justification for that? It will simply be a result of them having Muslims there. Simple.
Trying to talk about internal European problems as a way to be dismissive of this issue would be like me saying "well, my younger brother used to sometimes drink too much and break things in the house and take a swing at me from time to time, so why should I stop permitting one new violent crack addict stranger with a felony rap seet per week to move in from off the street?"
For starters, your brother is your brother. You don't get to choose who your family are, and to a certain extent you simply have to deal with their issues. Nothing's forcing you to let crack addict felons you don't know move in off the street. Secondly, their issues are far worse than your alcoholic brother's are and you're letting more of them in all the time.
I see people in this thread talking about what we do to stop this. No matter what your perspective is, or how abhorrent you find my views on how to stop this (I essentially wouldn't rule ANYTHING out to stop it, let your imagination go wild.) we should all at least be able to agree that step 1 is to STOP LETTING MORE MUSLIMS IN.