Brennan was born in Wexford. He was a journalist. During the Easter Rising of 1916, he held Wexford with 600 men, as commander of the Irish Volunteers. Sentenced to death, Brennan saw his sentence commuted to life imprisonment. He was released during the general amnesty in 1917. From January 1919, he worked at the Department of External Affairs of the parallel government Sinn Fein. For example, replacing Fitzgerald as Minister of Propaganda when he was imprisoned. After the Treaty of December 6, 1921, Brennan was committed to the Republican side during the civil war. He later refused the position of Secretary of the Department of Foreign Affairs. Brennan was the director from 1920 to 1934 of the newspaper created by de Valera, The Irish Press, and then joined the diplomatic service when Fianna Fáil came to power. He was director of Broadcasting from 1947 to 1948. He published various pieces, stories and his autobiography, Allegiance (1950).