1 - skywalker - In terms of mainland casualties inflicted, long term disruption, or 'terrorising effect' the IRA was not in the same league as the 9/11 attacks. For much of the time they sought to create low grade annoyance to inspire political change. 9/11 was much more about maximising body count. Furthermore, with a deliverable political objective you could also see a way out of the Irish mess. Much less clear in the current situation.
Absolute, complete and utter rubbish.
If they wanted to cause maximum loss of life they would have "smeared" NYC with the aircraft and killed far more people.
It was a symbolic attack, and there were ways to kill far more people.
As for your argument about the IRA, well the Americans decided to fund them for quite some time. I have absolutely no sympathy for the Americans, yes 11/9 was an awful event, but in the greater scale of things it really wasn't that bad.
How many innocent Iraqis have been killed ?
What goes around comes around......