Sargent was born 16 January 1890 at Manor Street in Waterford. She was the only child of Terence Hannan and the former Mary Daly. Her father died when she was an infant, leading her mother to move to England with her. They lived with her uncle, the Rev. Richard Hannan, at a parish at Lamley Moor, near Durham. There she was schooled in a French school and Loreto nuns. From a young age she was involved with the Gaelic League in London, possibly influenced by her cousin Richard Barry O'Brien. It was through her involvement in his movement she met her husband, Philip Armfield Sargent, a timber merchant. They married in London on 16 September 1908 when she was 17. Philip was an English Quaker, but had strong sympathies with Irish nationalism and was involved in the events of Easter 1916. After that he was possibly held in Lewes naval prison until June 1916 when he was transferred to Frongoch camp, north Wales.