The Irish Free State was created in 1922. During the subsequent Irish Civil War Bryan opted to join the National Army (later known as the Irish Army). He was commissioned to the rank of Captain in September 1923. He would remain in the Irish Army until his retirement in 1955.
For much of his career, he served with the Headquarters Staff, specialising in intelligence. In 1940 when a cipher was found on the first German agent to be captured in Ireland, Wilhelm Preetz, Bryan recruited Richard J. Hayes Director of the National Library of Ireland as a codebreaker and closely co-operated with him in the breaking of German codes.[2] In 1942 he succeeded Liam Archer as Director of G2; he exercised a decisive personal contribution towards the detection and arrest of German spies in Ireland, such as Hermann Görtz and Günther Schütz. Bryan remained head of G2 for the remainder of the War.
In 1952 he was appointed Commandant of the Irish Military College.
In 1983, RTÉ made a dramatised television series (Caught in a Free State) about German spies in Ireland during World War II. A character closely based on Dan Bryan – "Colonel Brian Dillon" – was played by the Irish actor John Kavanagh.
^McMenamin, Marc (2022). Ireland's Secret War: Dan Bryan, G2 and the Lost Tapes that Reveal The Hunt for Ireland's Nazi Spies. Gill Books. ISBN9780717192885.
^Dwyer, T. Ryle (2009). Behind the Green Curtain: Ireland's Phoney Neutrality During World War II (first ed.). Gill & MacMillan. p. 20. ISBN9780717146383.
Further reading
Carter, Carolle J. (1977). The Shamrock and the Swastika: German Espionage in Ireland in World War II. Pacific Books. ISBN9780870152214.
McMenamin, Marc (2018). Codebreaker: The Untold Story of Richard Hayes, the Dublin Librarian Who Helped Turn the Tide of World War II. Gill Books. ISBN9780717181612.
O'Halpin, Eunan (2003). MI5 and Ireland, 1939-1945: The Official History. Irish Academic Press. ISBN9780716527541.