The assumed existence of an extensive Irish fifth column had exerted a powerful effect on British policy towards Ireland during the summer of 1940. Fifth columnists would remain a serious threat so long as British military intervention in southern Ireland was a possibility: in the event of a German invasion they could provide crucial support to the invaders, divide Irish opinion and weaken resistance; if Britain sought bases in the south, they were a likely source of political opposition and violent attack. In addition, pro-Axis elements in Ireland constituted a pool of agents for German espionage, sabotage and propaganda operations against the United Kingdom – this threat persisted long after 1941. The British intelligence community set about investigating a wide range of individuals who might aid the enemy: the colourful cast of suspects included militant republicans, aspiring fascists, government officials, German diplomats and Abwehr agents. It took some time for British officials to get to grips with this threat. Although the wildest fears about the Irish fifth column and the IRA disappeared in late 1940, British intelligence officers continued to fret about Irish Quislings and the activities of the German legation. By 1942, however, the British intelligence community was less worried about Axis subversion and the activities of pro-Axis elements in Ireland. This was partly caused by reassuring developments in Ireland, partly by better British intelligence. Some of this intelligence came from an extensive covert network built up by SIS in southern Ireland. More came from the Irish authorities, either through the link between MI5 and G2 or from co-operation between the RUC and the Garda. After some problems in the middle of 1941, the Anglo-Irish intelligence relationship deepened considerably. For the rest of the war, Irish and British officials worked closely together to monitor and suppress any remaining threats to Allied security.

The dual intelligence approach

We have seen how the need for operational intelligence caused British intelligence agencies to undertake clandestine missions in southern Ireland in the summer of 1940. The panic about the Irish fifth column generated an even greater response.