O'Brien, Arthur Patrick Donovan (1872–1949), republican activist, was born 25 September 1872 in London, elder of one son and one daughter of John Francis O'Brien, merchant, from Cork, and Henrietta O'Brien (née Myers), from Kennington, Surrey, England. He was educated at St Charles College, London, and studied civil engineering in Paris and electrical engineering at Faraday House, London. He worked as an engineer in England, France, and Spain up to 1918.
In 1898, after his return from Spain, O'Brien joined the Gaelic League in London and, after holding various senior positions, was its president (1914–35). He also joined the Irish Republican Brotherhood and the Irish Volunteers. However, it was in 1916 that O'Brien came to be heavily involved in republican activities. Before the end of the Easter rising O'Brien founded the Irish National Aid Fund, and from then he was the president of the Sinn Féin organisation in England and Wales.
After the establishment of Dáil Éireann he served as chairman of the London Loan Committee and a representative of the dáil in England, and contributed to the spread of dáil propaganda as well as being a hub for the dáil and its representatives in Europe. During the period of the Anglo–Irish war, O'Brien worked closely with Michael Collins (qv) in the gathering of intelligence, and securing and shipping armaments to Ireland.
O'Brien was one of the founders of the Irish Self-Determination League of Great Britain (ISDL), and the president of the League's London district committee. At the official launch of the League (31 March 1919) he became its vice-president, and was responsible for most of the day-to-day activities of the League with Seán McGrath, his right-hand man. In London various meetings and demonstrations were organised by O'Brien, and they included a mass meeting at the Albert Hall (11 February 1920); a demonstration at Trafalgar Square (15 Aug. 1920) to protest against the British government prohibiting Daniel Mannix (qv), archbishop of Melbourne, from entering Ireland; and a demonstration during the hunger strike of Terence MacSwiney (qv) and his funeral in England. O'Brien was also involved in early preparation for the Irish Race Convention, held in Paris (21–8 January 1922). On this occasion he was one of twenty-two representatives from England, and was elected a council member of Fine Ghaedheal, a world organisation of the Irish, which was established during this conference.
O'Brien opposed the Anglo–Irish treaty, and while the ISDL at its annual conference (1 Apr. 1922) adopted, by a two-to-one majority, a neutral stance towards the treaty pending a general election in Ireland, he was elected the League's president at the second meeting of the adjourned conference of the ISDL on 29 July 1922, in the absence of the League's pro-treaty leaders. Meanwhile, on 18 April 1922 O'Brien was dismissed from his position as an envoy by the Department of Foreign Affairs of the provisional government. The direct cause of this dismissal was O'Brien's letter to the Irish Independent entitled ‘Political prisoners’, published on 17 April 1922, but it may have been a result of a strained relationship between O'Brien and the leaders of the provisional government. He continued his work of propaganda to raise support for the republicans, and campaigned for the release of Irish prisoners in British gaols. On 1 December 1922 he was reappointed as representative from the government of the Republic to England by Éamon de Valera (qv).
He was arrested in Dublin on 4 July 1922 but subsequently released. On 1 August he was arrested again in Dublin, and imprisoned in Maryborough jail until 21 August. On 11 March 1923 O'Brien was arrested again in London, deported to Ireland with more than 100 other prisoners, and interned in Mountjoy prison. Under a habeas corpus application, O'Brien was brought back to London and released on 16 May 1923, but was immediately rearrested. On 4 July 1923 he was sentenced to two years’ imprisonment for seditious conspiracy, and served at Wormwood Scrubs and Brixton prisons until his release on 23 July 1924.
Through his activities, O'Brien had become a spearhead of the Irish community in London. However, he was not immune to personality clashes or criticism. Charles Diamond (qv), a proprietor and the editor of the Catholic Herald and the Glasgow Observer, was his most prominent and persistent critic; O'Brien later sued him and was awarded £600. After the ratification of the treaty, O'Brien's attitude towards de Valera became embittered, mainly due to de Valera's idea of launching a new republican organisation in Britain. O'Brien's long imprisonment during 1923–4 contributed to a decrease of his influence among the Irish in England, and particularly in London. With a further split and decline of the ISDL, his activities tended to concentrate on the work of the Gaelic League while trying to maintain the organisation.
After his release, O'Brien took over the position of managing director of the publishing business of G. D. Ernest & Co. Ltd, and also became editor of their chief publication, Music Trades Review. In July 1935, under de Valera's Fianna Fáil government, he was appointed Irish minister to France and Belgium, and held this position until October 1938, when he retired at the age-limit. After spending some time in France, O'Brien returned to Ireland, and became deputy chairman of Mianrai Teoranta. He died 12 August 1949 at his home in Dublin. A collection of his papers is deposited at the NLI.