This thesis uses the life of Art O'Brien (1872-1949) as a central axis on which to construct an analysis of Irish nationalism in London from 1900 to 1925. Born and reared in London, O'Brien became a leading member of the Gaelic League, Sinn Féin, the Irish Volunteers, the Irish Republican Brotherhood and the Irish Self-Determination League of Great Britain. His career as an electrical engineer placed him at the centre of the London business milieu and he was a key mobiliser of the Irish community. Appointed London envoy of Dáil Éireann in 1919, he was a close confidant of Michael Collins throughout the War of Independence. He was also a mediator in various peace initiatives during 1920 and 1921 and introduced de Valera to Lloyd George at their first meeting in July 1921. Following O'Brien's rejection of the Anglo-Irish Treaty, he became embroiled in various financial disputes and suffered a spectacular fall from grace. He has been a neglected figure in the historiography of the Irish revolution and the Irish in Britain. Based on rigorous research of British and Irish archives, Irish language material and privately held papers, this thesis argues that O'Brien made a vital contribution to the prosecution of the Irish revolution by co-ordinating prisoner relief, financing gun-running and instigating a major propaganda campaign in favour of Irish independence. The dissertation demonstrates how a benign cultural nationalism gave way to militant activity for a small number of London enthusiasts who were galvanised by O'Brien. The importance of individual leadership rather than the influence of a movement is highlighted. This is the first comprehensive study of a London-based activist of the Irish revolution and also makes a significant contribution to the yet sparse historiography of leading second rank activists.