Yesterday in the IISS Library I found two additional resources on the issues raised. They are both online if your library is a subscriber.

'Northern Ireland and minimum force: the refutation of a concept' by B.W. Morgan and Prof. M.L.R. Smith, in the journal Small Wars & Insurgencies Vol. 27, No. 1 February 2016.

'Shadowboxing in the dark: Intelligence and Counter-terrorism in Northern Ireland' by Rory Finegan (linked to the Irish Army), in the journal Terrorism and Political Violence Vol.26, No.3 2016, a special issue in Irish Republican violence.