Brennan was born in John’s Gate Street, Wexford town in 1881. He joined the Irish Volunteers, and the IRB, and was responsible for helping to organise branches of the Gaelic League and Sinn Féin in Enniscorthy. He was a journalist for The Echo newspaper prior to the Rising. During the Rising he was one of the Commandants of the Volunteers in Enniscorthy. He was imprisoned for his involvement, and was sentenced to death. However, this was reduced to a sentence of penal servitude. He was released in 1917, but was imprisoned on a number of other occasions because of his activity in Republican politics. He was made director of elections for the Sinn Féin party in 1918. During his time in the Dáil between 1921 and 1922, he was under-secretary for foreign affairs. He supported the antiTreaty side in the Irish Civil War. He was one of the founders, and a director, of The Irish Press newspaper. He also served as a high-ranking diplomat in the US between 1938 and 1947. He died in Dublin in 1964.