Brennan, Thomas (1853–1912), nationalist and agrarian radical, was born 28 July 1853 at Yellow Furze, Co. Meath, second of two children of James Brennan of Beauparc and Catherine Brennan (née Rourke). Little is known of his earliest years or schooling, although he developed into a talented public speaker with a fine intellect. At the age of 18 he became a clerk in the Castlebar office of Murtagh Bros, the principal Dublin bakery, where his uncle James Rourke (1844–1921), later a prominent Land League official, also worked. Probably through the influence of Patrick Egan (qv), a friend of both Brennan and Rourke who worked in the head-office of Murtagh Bros, both men soon became members of the IRB. Acting under the orders of John O'Connor Power (qv), the Connacht IRB leader, Brennan and several other Mayo IRB activists campaigned successfully for Power's election to parliament in the 1874 general election amid strong clerical opposition. In 1876 Brennan and his uncle were transferred to the head office of Murtagh Bros by Egan, its manager, who purchased the company in 1878, renamed it the North City Milling Company, and made Rourke his business partner and Brennan his secretary. By 1877–8 Brennan was also civil secretary of the Leinster IRB and honorary secretary of the Dublin Mechanics Institute. In January 1878 he helped to organise a reception in Dublin to celebrate the release of Michael Davitt (qv) and of three other IRB prisoners. Thereafter Brennan, Davitt, Rourke, and Egan became lifelong friends.
After the launch of the ‘new departure’ programme in the Irish press in October 1878, they became enthusiastic about the idea of using the influence of the IRB to launch a large-scale land agitation in the west of Ireland; an ambition that, by 1880, had caused all of them to be expelled from the IRB. Together with Davitt and Egan, Brennan played a major role in organising a large protest meeting at Irishtown, Co. Mayo (20 April 1879), and was also a principal speaker at the Westport meeting (8 June) that led to the establishment of the Land League of Mayo. On the formation of the Irish National Land League (21 October 1879), he was appointed assistant secretary, while Davitt became secretary and Egan treasurer. These three acted as a hard-working triumvirate that virtually controlled the league's executive, although their power was reduced significantly after the creation of the Irish parliamentary party (May 1880), when C. S. Parnell (qv) insisted that the league's executive be expanded so it could include more moderate members. In 1879–81 Brennan attended more meetings of the central council of the league than any other significant figure in the movement. Meanwhile, at Land League rallies he became well known as an eloquent public speaker, compared by many to T. F. Meagher (qv) in terms of style and appearance. In his speeches he frequently linked the demand for peasant proprietorship with the Fenian demand for complete Irish independence, but also demonstrated radical, egalitarian views that were interpreted by some as evidence of socialist sympathies.
Between 28 December 1880 and 25 January 1881, together with thirteen other Land League officials, he was tried in Dublin for encouraging the non-payment of rent but was not convicted. Following the arrest of Davitt in Dublin (3 February 1881), Brennan took on Davitt's responsibilities as the league's national secretary and called for a general rent strike. On 23 May 1881 he was arrested under the protection of person and property act (1881) for incitement to riot and assault, and remained in prison for over a year. On behalf of himself and Davitt (then in prison in England), he signed the ‘no-rent manifesto’ that was issued from Kilmainham jail on 18 October 1881, and which led to the suppression of the Land League by Dublin Castle two days later. After his release from prison (June 1882), he sided with Davitt in opposing Parnell's policy of sidelining the land agitation. He also expressed a strong interest in Davitt's radical land nationalisation policy and encouraged discussion of this policy within the Young Ireland Society in Dublin, of which he became a vice-president. During the summer and early autumn of 1882, Brennan and Davitt accepted Parnell's invitation to help draft a political programme for a proposed new, national organisation in Ireland to replace the Land League. To their great frustration, however, on the formation of the Irish National League (17 October 1882), neither man was allowed by Parnell to play any significant role in its management.
Shortly afterwards Brennan left for New York, where he worked for Patrick Ford's (qv) Irish World, formerly the single greatest fund-collector for the Land League, which opposed Parnell and sympathised with the social radicalism of Brennan and Davitt. In March 1883 Brennan was reunited with Egan in New York and both men moved to Omaha, Nebraska, where Brennan, after a short time working as an attorney, became Egan's partner in the Omaha Mortgage Co., a successful firm of real estate, investment, and insurance brokers. By 1884 he was a popular travelling lecturer for the Irish National League of America. In his speeches, he was critical of Parnell's party and the National League agitation in Ireland for being too centred on Westminster, maintaining that they should concern themselves instead with educating the Irish masses in non-violent republican principles. Although a close friend of several Clan na Gael leaders (and quite possibly a member of that organisation), during the mid 1880s he denounced its ‘dynamite war’ against Britain, seeing it as damaging the efforts being made to advance republicanism in Ireland. On Egan's appointment to government office in 1889, Brennan took over the management of his business in Omaha and essentially committed himself thereafter entirely to business matters. For many years he was one of the most respected and well-known businessmen in the city. He never married, and died in Omaha on 19 December 1912.