O'Conor, Thomas (1770–1855), United Irishman and newspaper editor, was born 1 September 1770 in Dublin, second son of Charles O'Conor (qv) of Mount Allen, Co. Roscommon, who represented Sligo at the Catholic Convention of 1792 and died in America in 1808. His grandfather was Charles O'Conor (qv), antiquarian, author, and co-founder of the Catholic Committee, of Belanagare, Co. Roscommon. Educated privately, Thomas became a United Irishman on 13 January 1792. His role in the 1798 rebellion remains unclear but he left Ireland around 1801 and settled in New York, where he was an associate of William James MacNeven (qv) and Thomas Addis Emmet (qv). With William Kernan, he acquired a tract of land of over 40,000 acres in Steuben county and encouraged Irish emigrants to settle there.
He soon established a reputation among the Irish-American community as a writer. He established a weekly newspaper entitled War, which enjoyed a short run, and wrote A history of the revolutionary war in America, in which he highlighted the activities of Irish-American revolutionaries. He established another periodical in 1812, the Military Monitor, which also enjoyed a short run. In June 1814 he went into partnership with Edward Gillespy and took over the editorship of his newspaper, the Shamrock. The paper, which was primarily aimed at New York's growing Irish community, had been struggling, but its sales improved under O'Conor's editorship; it was his first really successful journalistic venture. The tone of the paper was vehemently anti-British and, as America was then at war with Britain, found a receptive audience. When British ships arrived off Sandy Hook in July 1814, he exhorted the city's Irish population to defend New York. Over 1,500 Irish people responded to his call and went to the Harlem river and Brooklyn Heights to help build fortifications overlooking the river.
In January 1815 Gillespy severed all connections with the newspaper and O'Conor took over as owner and editor. During the election for the governorship of New York in 1816, the newspaper achieved prominence owing to O'Conor's merciless attacks on the Federalist candidate, Rufus King. King had served as American minister to London (1796–1803) and had opposed allowing United Irish prisoners to move to America after 1798. O'Conor castigated him in the Shamrock, claiming that he was anti-Irish and blaming him for the death of Robert Emmet (qv) in 1803. He even enlisted the help of Thomas Addis Emmet, who wrote letters to the Shamrock in which he claimed that King had foiled his brother's plans of emigrating to America. Using phrases such as ‘Beware of an American King’, one editorial concluded: ‘if you don't vote against King you are a slave’ (McShane, 17). King polled badly in predominately Irish districts and lost the election. By 1817 the newspaper was faltering again, and O'Conor relaunched it as a periodical in 1819. In the same year he founded the Globe, finally winding down the Shamrock in 1824. The Globe followed a more general format in the hope of gaining a readership among the wider population of New York. He also published Selections from several literary works (1821) and The inquisition examined by an impartial observer (1825).
O'Conor was involved in various social activities on behalf of the Irish community, often acting in concert with MacNeven and Emmet. With them in 1816 he founded the Shamrock Friendly Association, which was closely aligned to DeWitt Clinton's Republican faction. From the offices of the Shamrock he advised newly arrived Irish emigrants about work and availability of land, and in his paper commended MacNeven's ‘Hints to emigrants’, available free from the Shamrock Friendly Association in 1811. O'Conor advocated the connection between economic opportunity and political liberty, and later sat on a committee that helped Irish emigrants buy land. He also sat on some of the committees of the Irish Emigrant Association, which was established by Emmet and MacNeven in 1817 to lobby Congress for land for emigrants, a campaign that was ultimately unsuccessful. In 1828 he was a co-founder of the Association of Friends of Ireland, which assisted newly arrived emigrants and collected donations for Daniel O'Connell's (qv) ‘catholic rent’ among New York's Irish community. He believed that by immersing themselves in American politics and culture and demonstrating the viability of democratic, republican institutions, the Irish would strengthen nationalist sentiments back home. Acting as the voice and champion of the Irish community, O'Conor was also a member of the Society of Friendly Sons of St Patrick, worked on their charitable committees, and was an organiser of the 1837 St Patrick's day celebrations. In 1841 he was a founding member of the Irish Emigrant Society, and worked tirelessly throughout the 1840s to improve the lot of the thousands of emigrants who arrived in New York during the famine. He died 9 February 1855 in New York.
He was married and with his wife Margaret had two sons and two daughters. His eldest son, Charles O'Conor (1804–84), was an attorney and politician. As special deputy attorney general for New York (1871–5) Charles O'Conor successfully prosecuted William M. ‘Boss’ Tweed for corruption. In 1872 he unsuccessfully ran as a third-party candidate of dissident Democrats in the presidential elections and was the first catholic ever to receive a presidential nomination.