Art O'Brien has been one of the mystery figures of the Irish nationalist struggle of the early 1920s. O'Brien is referred to repeatedly in histories of the Anglo-Irish War, as Mary MacDiarmada notes in Art O'Brien and Irish Nationalism in London, 1900–25, because his papers in the National Library of Ireland constitute the most extensive collection of correspondence between key figures in the conflict from 1919 to 1921. But who was Art O'Brien? He was not involved in 1916, not elected to the Dáil, not in the government, not executed by the British or the Irish Free State. MacDiarmada has written the first full biography of this Londoner who was active in, indeed central to, Irish affairs from 1900 to his death in 1949. O'Brien was born in comfortable circumstances in London in 1872 to an Irish father and an English mother and was well educated, studying engineering in Paris and working in France, Spain, and eventually England. He also kept in touch with his father's Irish family so that while he developed a successful electrical engineering consultative business he was by 1900 drawn into Irish cultural activities in London. He quickly became prominent in the Gaelic League in London, learning Irish and meeting key figures in the nationalist movement in England and Ireland. He joined the London branch of the Irish Volunteers in 1914 and helped to organize the acquisition of guns, a task that he continued throughout the Anglo-Irish War. Indeed, events increasingly drew O'Brien beyond cultural affairs into more political action. After the 1916 Rising, he joined the Irish Republican Brotherhood and worked actively to support Irish prisoners in British internment camps.

Following the Sinn Féin victory in the 1918 general election and the creation of the Dáil Éireann in 1919, O'Brien was appointed Dáil Envoy in London. Now working full time on Irish affairs, O'Brien headed the Irish publicity campaign in England, founding the Irish Self Determination League, working with the international press to keep Irish matters in the news, organizing rallies to protest events such as the death of Terrence MacSwiney, as well as manipulating the movement of money into and out of Ireland and facilitating the shipment of arms there. Indeed, he became “[Michael] Collins’ man in London” (196), as MacDiarmada says, with almost daily correspondence between them. MacDiarmada makes the argument that as the result of O'Brien's varied activities, London was a much more important diplomatic post than was Paris. She points out that by late 1920 and into 1921, O'Brien increasingly became a key link between the Dáil and Downing Street. In July 1921, it was O'Brien who introduced Eamon de Valera to David Lloyd George. However, with the beginning of the Anglo-Irish talks, O'Brien's influence began to diminish. He was not included in, or consulted by, the delegation headed by Arthur Griffith and Michael Collins (despite his closeness to Collins), a slight that he resented. Of course, he was oblivious to the fact that he had not been elected to the Dáil nor was he a member of the government.

O'Brien concluded that the Irish had been outmaneuvered by the British in the negotiations, and he opposed the Anglo-Irish Treaty immediately. He steadily backed de Valera and the anti-Treaty faction in the Civil War. Working in opposition to the Irish Free State he was arrested in London in 1923 and deported to Dublin, giving rise to a celebrated court case based on a 1679 British Habeas Corpus Act, which he won, based ironically on the assertion that the Irish Free State was a separate jurisdiction and not, as the anti-Treaty forces claimed, still part of the United Kingdom. O'Brien was of course dismissed as the Dáil Envoy in London, and having given up his engineering firm in 1919 had to support himself through the old family business of publishing. However, when de Valera came to power again in 1932, O'Brien, along with a number of other old republicans, was returned to government service. With Sean T. Ó Ceallaigh's support, O'Brien was appointed Irish Minister to France from 1935 to 1938. He retired to Dublin where he died in 1949.

MacDiarmada provides a balanced assessment of O'Brien, a central figure in Irish affairs in London in a most critical time. She acknowledges his energy, devotion, and administrative skills. His business and international connections, his language skills, and his personal knowledge of Irish political figures, made him very effective in promoting Irish needs. O'Brien got things done. But she admits that he was, “a perfectionist who did not suffer fools gladly and who had a highly inflated sense of his own importance” (200). In short, he was difficult to work with and made enemies. He was certainly distinctive in being someone not actually native to Ireland who played such a central role in Irish affairs for almost forty years. MacDiarmada has brought into the light this shadowy figure and in doing so has explained the importance of both Art O'Brien and the London Irish in the history of the Irish nationalist struggle.