The NSA's efforts against Russia and China and other geopolitical enemies of the day are probably not much influenced by wholesale surveillance of American citizens. If the question is what the NSA as a whole is doing, the answer is, "a lot". I was concentrating on this one particular bit.

As far your IRA example, the US doesn't face anything like that, and shows no prospect of doing so. Even if it did, I'm not convinced that efforts like the NSA's would be very helpful in combating it, and given the example of the last 12 years, I'm quite sure that the US government's reactions as a whole would be far more damaging than the terrorism itself.