This may be where I part company with Adam Curtis. Yes, espionage and counter-espionage is a game of shadows. Sometimes there may not be anything lurking there at all. There undoubtedly was a group of hard right paranoiacs in Mi5 who lost the plot in 1970s; I suspect they never really understood the Soviet Union very well.
But in Ireland (and against the left and the trade unions) the security services were not chasing anyone's tail. They understood the 'threat' all too well, and had it under surveillance, infiltrated, and neutralized by the late-1980s. And many people - guilty, compromised or innocent - were killed.
Shooting of Bernadette Devlin
Shooting of Bernadette Devlin
Surveillance operations in South Armagh