In no way am I justifying the IRA's bombing campaign, but their actions were / are rooted in centuries-old bad blood between Ireland and Britain. Remember that the province of NI only exists because it was annexed by invading English Protestants from the mainland and that thousands of inhabitant Catholic Irish were either killed or displaced in the 1600s and 1700s, just like thousands of inhabitant Scots were killed or displaced by the invading English armies under Edward I and the thousands of inhabitant Welsh were killed by the English armies under Edward II. Wales and Scotland, fortunately, retained their identities. NI's indigenous Catholics are struggling to assert theirs and, through organisations like the Provos, used violence.
The only solution is to abandon any semblance of Britishness or Irishness, Protestantness or Catholicness, Unionism or Nationalism. Which would be like asking an American to burn the Stars and Stripes.
The IRA's terrorist campaign was brutal and unnecessary. Similarly the UDF, LDV and other loyalist organisations' "enforcement" of the Protestant occupancy is totally antagonistic and provocative. The antiquated Orange Order parades are completely without merit and act only in the same, lording-over-the-manor fashion. The Protestants are rubbing the Catholics' faces in the dirt. NI is a societal quagmire, where everything that is wrong about religious diversity and social difference combust in a giant cauldron of suspicion, hatred and division.
Moving onto 9/11, America's foreign policy had given enough countries and societies in the world reasons to be resentful or harbour hatred. The unnerving support of another invaded, annexed land - Israel (Palestine) - causes friction in the very heart of the Middle East. However another feeling - envy, jealousy - also compounds the problem. America is everything the extreme Muslim hates - and wants to destroy.
9/11 was not deserved by America, but America was a cause of it. There is an important differentiation here. Whilst the root of a problem may be as guilty as anything else of causing the event that follows, an act of retaliation such as 9/11 is wholly undeserved. It merely compounds the problem and leads to the situation we have today - a polarised world distrusting each other with equal regard. The IRA bombings were not deserved by England, but England was a cause of it.
Therefore my conclusion is that 9.11 and the IRA campaign are to be held in equal regard, contempt, sadness and anger.

Edit - The governments cause the problem, the people suffer because of it. That's pure militant Socialism. Scary.