In 1988 Denis Kennedy published his ‘The Widening Gulf: Northern Attitudes to the Irish Free State 1921-48’. Kennedy clearly demonstrated - more decisively than any other writer - that unionist attitudes in the North were seriously affected not only by the nationalist violence directed against Southern Protestants in the period 1920-22, but also by their general treatment thereafter as Dublin moved ever more explicitly towards the adoption of the Catholic constitution of 1937...
These new security related papers (both of these in Kew and Belfast) reveal that there was rather more reason for unionist paranoia in the early 1920s than has traditionally been acknowledged by historians. Let us consider the nature of IRA violence throughout Ireland in the 1919-21 period. The new Public Records Office (PRO) files, released in November 1996, support the recent thesis of Peter Hart (and go rather against the comments in Campbell 1995, on this point) that IRA violence was much more sectarian than has been acknowledged by scholars.
Campbell advances some evidence that implies that a southern protestant willingness to act as British “spies” might explain their casualty rate. Hart disagrees and is supported by the most important of the recently released files. Referring to the execution of so many so-called spies, the final ‘end of term’ lengthy report on the work of the British intelligence services in Ireland makes it clear that the Southern Protestant community has long since been frightened into silence; in consequence it provided little information. This document offers a different explanation for the IRA’s modus operandi:
“Numbers of ex-soldiers and others have been murdered during the rebellion, not so much because they were discovered in active espionage - indeed, few of these had ever given information - but they met their deaths partly because there was a possibility that they might become informers and partly in order to keep alive the reasons in which it was considered desirable to impose. The outside public knew not whether or not the man foully done to death was an agent or not. In the customary notice found pinned on his back, it was inferred that he was, and, when cases of this kind were numerous, the layman concluded that the rebel organisation has almost miraculous facilities for tracing a betrayer.”
...(James Craig) signalled that this community should rally to the support of the Free State government as the least unpleasant available option - but he also argued that the North should signal a possible long-term willingness to come in with the South, in order to protect that community from republican onslought in the South. In later years, as the southern protestant community went into rapid decline by the end of the 1920s - as a result of policies of violence, ‘control’ through language policy and breaking of professional links with the rest of the UK - this consideration ceased to have a significant impact on Craig approach.”
- Paul Bew, 1999, here: https://www.thebritishacademy.ac.uk/sites/default/files/98p401.pdf