Ermm....try the fact the IRA lost its main finance/support network after 9/11 when the US population had a taste of what terrorism actually means...
That said...basically, in the case of Ireland, terroism worked
Accepting that the the causes of both the start and end of the Troubles are more complex than any single sentence you or I could write, I think you'll find the Good Friday Agreement pre-dates 9'11 by several years.
I have no doubt that any argument suggesting that the British Government really stood by some public stance of not talking to terrorists is utter rubbish but there is no doubt that the perpetuation of extreme violence to support either end of the idealogical spectrum lost it's appeal over time to a greater proportion of the population.
Of course concessions were made. The inclusion of Sinn Fein in the political process and into Stormont was critical in seeing the end of the violence. You can argue terrorism worked however there was no unification of Ireland. There was no complete delivery by the IRA of their ultimate aims. If Northern Ireland needs defending it will be the British Army that does so. The majority of Unionists lost interest in listening to the moronic ramblings of Paisley and Adams and McGuinness had to present an acceptable political face rather than be seen as the puppets of a bunch of paramilitaries.
The extremism of ISIS leaves no room for such compromise. Military action may be the only way of containing the problem for the moment but ultimately it will need solving by communication and hopefully the moderate voices shall start to gain traction at some point.