Britain's intelligence relationship with Ireland had come a long way since 1916. From a rebellious territory, the twenty-six counties had evolved into a state with de facto independence. For over two decades the British intelligence system had found it uniquely difficult to handle this transition. This had contributed to frequent reversals of British policy. However, by the middle of the Second World War Britain had developed mature, sophisticated mechanisms for collecting and using Irish intelligence. This not only laid the foundation for wiser policy-making; it also helped bring about a more fundamental change in British perceptions of Ireland.

Poor intelligence had been one of the reasons for Britain's capitulation to Sinn Féin and the loss of southern Ireland during the revolutionary period. The old Irish system, organised around the RIC and DMP, struggled to penetrate republican inner circles before 1918, and was then smashed by an audacious IRA guerrilla campaign. New British agencies and personnel intervened in Irish affairs in a disjointed, ignorant and amateurish way. They lost the covert intelligence war and instead had to resort to overt, indiscriminate intelligence methods, which, though yielding information, alienated the Irish population. Of course, the British intelligence system was also hampered by the misguided decisions of policy-makers in Dublin and London, and the frustration of much of the Irish people at the failure to introduce self-government. The importance of this unfavourable political environment was illustrated by Britain's greater success when dealing with the republican movement outside the twentysix counties: in Northern Ireland, the British could call on the support of the unionist majority and its new government, which quickly took over responsibility for internal security; republican activities in Britain and abroad were tackled more easily by Britain's established domestic and foreign intelligence system (especially when the new government in Dublin agreed to co-operate on security matters during the Civil War). Yet, as well as suffering the consequences, the British intelligence system bore some responsibility for causing the political failures.