Reply #13: It's a bit chancey to assume it was IRA

[Maeve]

And there are several arguments against it.
First, they have denied involvment. (I know, I know) Second, they are on stand-down; any action taken would be seen from the outside as proof that they are undisciplined, the opposite of what you are arguing. Third, it would be politically stupid in the extreme, especially after Gerry Adams has made a point of saying that Donaldson was under no republican threat. It would also be proof of criminality, a charge the IRA is trying to shed itself of.

Odds are better that it was 1) a dissident, probably anti-GFA, republican with a hatred for spies (remember he worked for the British for 20 years) or 2) one of his ex-handlers cleaning up loose ends.