This book provides the full text of the history of MI5's Irish section BIH, a secret document prepared in the immediate aftermath of the Second World War. Declassified only in 1999, the history gives a detailed account of the establishment and work of BIH, including its crucial liaison with Irish army intelligence. In addition to providing much fresh material on German espionage involving Ireland before and during the war, the history casts much new light on the counterintelligence and codebreaking activities of other British secret agencies - MI6, RSS and GCHQ- as they sought to track down and to thwart Axis espionage and subversive operations conducted against Britain through neutral Ireland. It also provides fresh information about the organisation of British intelligence in both parts of Ireland before and immediately after the war, and it includes frank appraisals of Irish security and counterintelligence methods, personalities and policies.