Birthplace plaque, Thomas Davis Street, Mallow, Co Cork
William O'Brien was born at Bank Place in Mallow, County Cork, as second son of James O'Brien, a solicitor's clerk, and his wife Kate, the daughter of James Nagle, a local shopkeeper. On his mother's side he was descended from the distinguished Norman family of Nagles, long settled in the vicinity of Mallow giving their name to the nearby Nagle Mountains. He was also linked through his mother with the statesman Edmund Burke's mother's family, as well as with the poet Edmund Spencer's family.
The Nagles however, no longer held the status or prosperity they once had. In the same month thirty-eight years earlier Thomas Davis was born in Mallow. O'Brien's advocacy of the cause of Irish Independence was to be in the same true tradition of his esteemed fellow-townsman. Following in his footsteps he acknowledged the existence of many strands of Irishness.
O'Brien shared his primary education with a townsman with whom he was later to have a close political connection, Canon Sheehan of Doneraile. He enjoyed his secondary education at the Cloyne diocesan college, which resulted in his being brought up in an environment noted for its religious tolerance. He greatly valued having had this experience from an early age, which strongly influenced his later views for the need of such tolerance in Irish national life.