He was born at Kilrush, County Clare. He studied law at the Catholic University, Dublin, after which he went to London. He was a founder-member there of the Irish Literary Society and also joined the London Gaelic League. He became political secretary to Patrick McMahon, a role which introduced him to senior British and Irish politicians.
He was loyal to, but not uncritical of, Charles Stuart Parnell, leader of the Irish Parliamentary Party at Westminster. Parnell wished to make him a MP, but he declined, as he preferred to remain focused on writing. He wrote a much-discussed biography of Parnell in 1898. O'Brien was a political insider and a committed Home Ruler and the biography throws light on the activities of Home Rule MPs and their links to the Fenian Movement.
He supported involvement in the First World War, where three of his sons fought.
He died in London and was buried in Glasnevin Cemetery, Dublin.