O'Brennan attended the inaugural meeting of
Cumann na mBan, held in
Wynn's Hotel, Dublin, and was a member of its central branch. When Cumann na mBan split due to the start of
World War I the O'Brennan's sided against the proposal to volunteer with the British. Both sisters were involved in planning for the
Easter Rising. O'Brennan bought the makings of the flag for the 4th Battalion of the
Irish Volunteers and made the first aid kits. She and her sister, Áine Ceannt, helped to assemble equipment and she carried dispatches for
Éamonn Ceannt, her brother-in-law. Like many she only discovered the mobilisation was going ahead the night before it happened. O'Brennan was stationed in the Marrowbone Lane distillery under the command of
Con Colbert and remained there all week. After the Rising, in late 1917, O'Brennan joined the executive of Cumann na mBan, she was also a member of the
Sinn Féin executive. In 1918 she recruited new members throughout
Longford and
Wicklow. O'Brennan was one of the two delegation secretaries for the 1921 treaty delegation.
Mary MacSwiney proposed O'Brennan to be appointed as a plenipotentiary to the negotiations but she was thought to be "too extreme". She lived in 22 Hans Place, Brompton, London and worked for
Arthur Griffith as his secretary for a time in 1922. Despite being part of the
Treaty group, O'Brennan was on the
Anti-treaty side during the
Civil War. She worked on the staff of the Republican headquarters, secretary to
Erskine Childers. She was arrested there in November 1922 and imprisoned in
Mountjoy Prison, then moved to Kilmainham on 6 February 1923 where she spent three months. During this time she participated in the
1923 Irish hunger strikes. It was there she organised the 7th anniversary commemoration of the Easter Rising. In May 1923 she was sent to the
North Dublin Union, where she took part in a failed escape attempt. She was however released only a few weeks later. O'Brennan lived in Churchtown, Dundrum, County Dublin. She died on 31 May 1948 and was buried in the
Deansgrange cemetery. ==Further reading ==