"My money would be on the possibility that it was the S.A.S., deliberately staged to discredit Sinn Fein and the I.R.A"

The SF and IRA have done a good job discrediting themselves at it is. I don't think they required help from the Special Air Service.

A similar sort of thing happened in Quebec when English language rights were taken from the English speakers of the province. Canada stood by and allowed it to happen. Approximately 800,000 English speakers had their language rights taken away. So, while I do make an effort to understand the Troubles--a process started by the British, I fail to see that Irish killing Irish brings the difficulties to a close or to a satisfactory conclusion.