Seán MacEoin was born in County Longford in 1893. He worked as a blacksmith and a a farmer but came to prominence in the War of Independence as leader of a flying column. He held the village of Ballinalee agains superior British forces in 1921. He was later captured, imprisoned and sentenced to death. While in prison he was elected to Dáil Éireann in the general election of May 1921. MacEoin was eventually released from prison after an ultimatum from Eamon de Valera. In the debate on the Treaty MacEoin seconded the motion that it should be accepted.
MacEoin joined the Irish Free State army as a senior officer. He was appointed Chief of Staff in 1928. In 1929 he was elected to Dáil Éireann as a TD. From 1948 to 1954 he served as Minister for Justice. From 1954 to 1957 he was appointed Minister for Defence. MacEoin stood unsuccessfully as Fine Gael candidate for the Presidency in 1945 and 1959. MacEoin retired from politics in 1965.