[1] O'Brennan was born in Summer Street, Dublin, the third daughter of Francis Brennan, auctioneer, and Elizabeth Anne Butler.

[7] 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.

O'Brennan bought the makings of the flag for the 4th Battalion of the Irish Volunteers and made the first aid kits.

[7][8][9][10] O'Brennan was stationed in the Marrowbone Lane distillery under the command of Con Colbert and remained there all week.

O'Brennan worked as a clerk for the National Aid Association later the Prisoner's Dependents Fund as well as locating and marking the graves of those killed during the Rising.

[7][17][18][19][8] O'Brennan wrote a novel, using the pseudonym ‘Esther Graham’, entitled The call to arms: a tale of the land league (1929).