Born in Co Mayo, she was the daughter of a Church of Ireland rector and related to Countess Markievicz. Lynn qualified as a doctor in 1899 and in 1904 set up a practice in Rathmines, Dublin. She was was a feminist, socialist and nationalist who assisted hunger-striking suffragists.
In 1913 she became a support of the labour movement and subsequently joined the Irish Citizen Army. She was stationed at City Hall during the Rising and was chief medical officer. Lynn later practiced as a doctor during the War of Independence and in 1919 established the first infant hospital in Ireland, St Ultan’s, with her friend Madelaine french-Mullen.
For a period, the establishment was entirely staffed by women. In 1955, Lynn was buried with full military honours for the role she played in the Rising, at Deansgrange Cemetery, Dublin.