O'Brien, Michael (1836–67), Fenian, was born in Ballymacoda, in the parish of Kilmacdonagh, Co. Cork, son of a substantial farmer. Educated locally, in about 1850 he was apprenticed as a draper's assistant to Arnott, Grant and Co. of Youghal and later moved to The Queen's drapery, Old Castle, Cork city. In 1856 his family were evicted from their farm. O'Brien was secretary of a national reading room in Cork city around 1858–9, and was sworn into the IRB through his friendship with the former Phoenix Society leaders in Skibbereen, such as Jeremiah O'Donovan Rossa (qv). For three years he served as deputy to Brian Dillon (qv) in the Cork IRB. Having lost his job in December 1861, five months later he decided to follow Fenian friends to America, some of whom had already become officers in the US army. In July 1863 he joined the 13th New Jersey regiment and later served in the 10th Ohio regiment alongside fellow Fenians such as Ricard O'Sullivan Burke (qv) and Thomas J. Kelly (qv).
He was mustered out of the army having attained the rank of lieutenant and US citizenship, and in late 1865 he returned to Ireland, finding work as a shop assistant in Cork city. Over the next year he procured arms for the Fenians in Ireland and England under the direction of O'Sullivan Burke, travelling regularly between the two countries. In late 1866 he was arrested in Liverpool with three other Fenians on charges of unlawful possession of government arms, the city constabulary having unearthed a cache of army rifles in the cellar of a house which they used. However, the evidence against them was slight and all four were acquitted in November 1866. O'Brien returned to Cork and secured employment at the Munster Arcade, Cork city, where he remained until late March 1867, when he vanished from public sight for a time, having roused the suspicions of the Irish constabulary after the rising of 5–6 March. It appears that he had been one of a party of some hundred men which, having assembled on the northern outskirts of Cork city on the night of 5 March 1867, attacked the constabulary barracks at Ballyknockane, and uprooted rails and sleepers on the railway line later that night.
Escaping to England he stayed until September 1867 with Edward O'Meagher Condon (qv) at a cheap lodgings in Shudehill market, Manchester, under the assumed name of William Gould. He was one of the principals involved in planning the rescue of the Fenian leaders Timothy Deasy (qv) and Thomas J. Kelly as they were being transported in a police van on 18 September. Assigned to cover the retreat of the rescue party, O'Brien was distracted by trying to disperse spectators who had gathered beforehand to watch the rescue (an open secret in the Irish community), and was arrested as he tried to flee the scene. He and three accomplices – William Philip Allen (qv), Michael Larkin (qv), and Condon – were charged with the murder of police sergeant, Charles Brett, who had been shot dead during the rescue. Before the trial O'Brien sought help from the American consul but received a chilly response from an administration anxious not to jeopardise Anglo-American relations by intervention in British judicial proceedings. Delivering a speech after his conviction for murder on 20 November 1867 O'Brien criticised the failure of the US government to protect him from an unjust trial. He vigorously denied the charges and claimed that he had been improperly identified while in custody, having been conspicuously manacled in view of the witnesses. He was hanged in Manchester on 23 November 1867. Like Larkin, he did not die instantly but had to have his legs pulled to break his neck. Physically the most impressive of the four convicted men, he supported Allen on the scaffold. He never married.