Seán O'Casey was born March 30, 1880 in Dublin as John Casey, the son of Protestant parents Michael and Susan (Archer) Casey. After his father's death when Sean was six years old, the family moved around Dublin a great deal. Sean left school at the age of 14 and worked odd jobs, including several years as a railwayman on the Great Northern Railway.

In 1906 O'Casey joined the Gaelic League and began learning Irish and the Uilleann pipes. He also joined the Irish Republican Brotherhood and was briefly a member of the Irish Citizen Army.

O'Casey began writing seriously in the 1910s, and his first play, The Shadow of a Gunman, was performed at the Abbey Theatre in 1923. This was followed by several other plays over the next three decades, many of which were controversial and not popularly received, though other notable playwrights often responded favorably to his works.

O'Casey married Irish actress Eileen Carey Reynolds (1903-1995) in 1927. They had three children: Breon, Niall (who died of leukemia in 1957), and Shivaun. Sean O'Casey died September 18, 1964.