>>I suspect that one plausible answer lies in the way the UK dealt with the IRA's methods- merciless, ruthless pursuit and vigilance, but invading Ireland was never a serious thought.

Of course it wasn't any sort of vigilance that took down the IRA. It was prosperity plus liberal ideology that religious sect doesn't determine your worth as a person. Prosperity came with the EU plus taxation reform (reduced taxation) that drew Intel and other firms into Ireland, creating professional career paths that propelled everybody on a positive upward trajectory. The IRA could not hope to recruit youths with a choice of jobs with opportunity to double salary by undertaking some extra training, with sights set on a car and big TV for ma and pa by end of year. Terrorism thrives on resentment and unemployment with inverse relationship to prosperity, something the West would do well to consider when reacting to illegal immigration, Syria et al.