British Intelligence during the War of Independence was far too fragmented for its own good, with "spies" being inserted from at least 3 sources (Military, Dublin District Special Branch, Thomson) and as well as the RIC.
The Scotland House organisation - Towards the end of 1919 it was decided by the Irish Government that the secret service in Ireland should be controlled from London and directed from the office of Sir Basil Thomson, in Scotland House. It was believed that agents could be collected in England and sent to Ireland more discretely than if the head office were in Ireland where it was difficult to ensure secrecy. The military authorities in Ireland agreed that with the exception of any agents already empIoyed by the Intelligence Branch at GHQ, all agents should be controlled from Scotland House, and an officer from Scotland House was attached to GHQ for liaison duties,
Smith-Cumming and SIS (then MI1(c)) organized a new espionage unit in Ireland, called the Dublin District Special Branch, in mid-1920. The DDSB consisted of some 20 line officers drawn from the regular army and trained by Smith-Cumming's department in London. Beyond that, however, Smith-Cumming began importing some of his own veteran case officers into Ireland from Egypt, Palestine and India, while Basil Thomson organized a special unit consisting of 60 ethnically Irish street agents managed from Scotland Yard in London.
Dublin Division Special Branch worked under what one might call "shallow cover" (the IRA knew pretty well who they were), others from Basil Thomson who worked under deep cover and were mainly "ethnic Irish"
Thomson is believed to have sent about 60 agents to Ireland. Winter claimed that only 1 of these was murdered, so good was their training - that seems unlikely to me. They were independent of the army and reported back to London - hence communication took time. They used invisible ink, the formulae for which were only declassified in recent years.
Other agents were more successful. One quoted at length in Narratives; [British Intelligence in Ireland 1920-1921; The Final Reports-edited by Peter Hart] led a raid that captured 3 senior IRA members writing communiques to their subordinates. He rewrote the messages summoning all IRA leaders in the district to a meeting where they were arrested. Maintaining his cover as an IRA member he was placed into custody alongside them, gaining more intelligence from their conversation in jail.
Thomson's papers in London have never, and I suspect will never, come into the public domain. Even when a man was murdered, it is difficult to know for sure if he was a planted spy from London, or a low level man just picking up the odd few pounds for selling information.
On Sunday, 21 November 1920, the Headquarters Intelligence Staff of the IRA, and its special Counterintelligence Branch (known as "The Squad") under the leadership Michael Collins, mounted a successful operation to some of Smith-Cumming's case officers. In fact only 5 of the 15 men shot that morning appear to have been Intelligence, the rest having a mixed bag of jobs in Ireland. Many more appear to have escaped the IRA execution squads that morning.
Men that I think are Thomson men include the following. But at the same time one has to avoid labelling "nutters" as "spies" though many may have been.
These men are distinct from the Dublin District Special Brance men who were trained in the Royal Fusiliers Spy School at Hounslow. For example
Dublin Castle Intelligence