O'Brien, James Nagle (1848–78), Fenian, was born at Mallow, Co. Cork, the eldest of three sons and one daughter born to James O'Brien, a solicitor's clerk, and Kate, daughter of James Nagle, a prosperous Mallow merchant. The home was a nationalist one: he was a brother of William O'Brien (qv) (1852–1928) and their father was a Young Irelander. Educated at the Mallow national school, he began studies about 1865 at St Colman's College, Fermoy, with the intention of entering the priesthood, but returned home after six months and joined the Fenian movement. Apprenticed to a blacksmith, O'Brien spent his spare time writing nationalist poetry, drilling with local Fenians and casting bullets in an old corn mill. He also avidly sought to keep his younger brother William out of the Fenians in Mallow. As ‘Shamus', he contributed songs to the Irishman and the Shamrock, some of which became popular, especially in Cork. A popular figure with a fine physique, he was the model for the hero ‘Ken Rohan’ in William O'Brien's novel When we were boys (1890).
He became a local Fenian leader and took an active role in the March 1867 rising at Mallow. Prospects did not bode well for the insurgents; lacking weapons, they raided a shovel factory and armed themselves with pitchforks and scythes. O'Brien's revolver was one of the few working firearms they had. They captured and torched the local police barracks, although O'Brien helped the police sergeant's wife and child escape from the burning building by a ladder. He was arrested at home the following evening on a warrant for treason and imprisoned in the Mallow bridewell, but was eventually released for lack of evidence after the sergeant's wife refused to identify him as one of the men at the barracks that night.
Over the next few years O'Brien was involved in securing guns for Cork Fenians: he assisted in seizing two hundred rifles from the Cork militia's armoury, and in taking arms from the Martello tower at Fota and from Cork city gunsmiths. He took part in J. J. O'Kelly's (qv) arms-smuggling operation between Britain and Ireland (1869–70), and on at least one occasion enlisted the aid of his brother William. During the 1870s he worked for a firm of shipping agents, but grew ill by 1878 with tuberculosis. He died 14 December 1878. His youngest brother Dick died the following day, and his sister Maggie died 5 January 1879 – all from tuberculosis.