He joined the Gaelic League and the Irish Volunteers and was recruited into the IRB by Seán T. O'Kelly.

He married Úna Brennan who was also active in the republican movement and had her sworn into the IRB.

He commanded the insurgents in Wexford during the 1916 Easter Rising, with approximately 600 men in the town of Enniscorthy.

With Commandant Paul Galligan operating in the surrounding rural areas, almost all of north Wexford was in rebel hands.

Brennan claimed that the Bulletin was so successful its (unsuccessful) suppression became a major objective of the British Military Government.

In her story The Day We Got Our Own Back she recounts her memory of how, when she was five, her home was raided by Free State forces looking for her father, who was on the run.

Robert, his wife, and one of his sons returned to Ireland (his three daughters remained in the United States) when he was appointed Director of Radio Éireann (1947–1948).