Brendan Finucane was born on 16 October 1920, the first child of Thomas and Florence Finucane of 13 Rathmines Road, Rathmines, Dublin, Ireland. His mother was English, originally from Leicester. Her mother had travelled across Canada at a young age, and Florence moved to Dublin to seek her own adventure. She accepted the risks associated with living in the city. In 1919, she met Thomas Andrew Finucane, who had been involved in the Irish Rebellion. Thomas Finucane had been taught mathematics at college by Éamon de Valera, leader of the Irish opposition. As a member of the Irish Volunteers, he served under de Valera's command in the 1916 Easter Rising in Dublin. Thomas' father, Brendan Finucane's grandfather, was an Irishman who had served in the King's Own Scottish Borderers on the North-West Frontier Province. Thomas Finucane had ceased his political activism shortly before the couple married in October 1919, and after Florence had converted to Catholicism.