Britain's traumatic experience of the Irish revolution reinforced preconceptions about Ireland that would influence British policy decisions until the 1940s. Preconceptions are important to any understanding of intelligence and its role in policy-making. Policy-makers make decisions not on the basis of objective analysis of the facts, but by employing subjective cognitive premises and belief systems. They see what they want to see, select the information that fits their pre-existing hypotheses and biases, and ignore what is inconvenient. In his analysis of notable intelligence ‘failures’ of the twentieth century, Richard Betts concludes that ‘the ultimate causes of error in most cases have been wishful thinking, cavalier disregard of professional analysts, and, above all, the premises and preconceptions of policy makers’. The role of preconceptions is especially powerful when information is incomplete, contradictory or ambiguous, or when the subject is inherently mysterious and unknowable. The most that an effective intelligence system can do is to educate and enlighten government leaders, shaping their cognitive premises and challenging obvious errors, so as to minimise distortions.

This is the case today, even in those countries with sophisticated intelligence systems. Preconceptions had an even greater influence on British perceptions of Ireland in the 1920s and 1930s. At that time the British intelligence system was in a ‘pre-modern’ stage of evolution. There was no system for collating, assessing and synthesising the many different pieces of information flowing into the government. Professional intelligence agencies were neglected. Prejudices and assumptions distorted the British government's understanding of many other countries: for example, ‘faulty expectations’ and ‘preconceptions’ were at the heart of Britain's failure to accurately assess the German threat in the 1930s. Preconceptions were especially important in relation to Irish affairs. Ireland was so closely connected to Britain, and had been an emotive political issue for so long, that most British leaders had strong opinions on its people, its problems and the right solutions. The informality of British intelligence on Ireland meant that these prejudices and assumptions were not systematically challenged.