Underwood, Thomas Neilson (1830?–1876), founder of the National Brotherhood of St Patrick, was born probably at Strabane, Co. Tyrone, the eldest son of William Robert Underwood, and his wife Margaret (née Neilson). According to a lengthy obituary of Thomas Neilson Underwood in the Ulster Examiner (the main source for his life), his father was ‘descended from an old Wexford family’ and his mother ‘the daughter of one of the oldest merchants in Strabane’. His maternal grandfather, a Volunteer and United Irishman, was ‘allied by consanguinity’ to the United Irish leader Samuel Neilson (qv), whom he sheltered at Strabane when he was evading arrest in 1798. Underwood's family emigrated to Australia leaving him, a delicate child, to be reared and educated at Strabane by his aunt, Esther Neilson, who passed on to him her father's radicalism and her memories of the rebellion of 1798. Underwood seems to have lived at Strabane all his life, though he may once have studied at Edinburgh and he read law in London. He read widely and was influenced by the Nation newspaper. His early political sympathies came to lie with the Repeal and Young Ireland movements, a consequence of which was that his aunt's house was searched by police for arms and papers (1848).
He was president of the local Total Abstinence Association and when, in 1849, the Strabane Tenants’ Protection Society was formed, he was elected secretary. In this capacity he attended the inaugural meeting in Dublin of the Irish Tenant League (6 August 1850). Underwood threw himself into the work of the national body and was later recalled by one of its principals, Charles Gavan Duffy (qv), as being ‘in appearance a sickly sentimental boy but with a manifest reserve of passion and will’ (League of north & south, 51). During the 1850s he acquired a reputation as a speaker and writer on land questions. He believed not only in statutory recognition being given to the rights of tenant farmers but in protection being given to cottiers too. At the parliamentary elections held in 1852 he supported candidates favourable to tenant-right in Counties Donegal and Tyrone. Underwood did not follow William Sharman Crawford (qv) and James MacKnight (qv) when they left the Irish Tenant League. He began the study of law, was admitted to the Inner Temple and was called to the English bar (1860), but he did not practise and was refused admission to the Irish bar owing, it was said, to his political activities. During the 1850s he imbibed certain ideas about freehold which he elaborated into a plan for peasant proprietorship published in the Nation on 5 and 12 December 1857. On his return to Ireland, in addressing a public meeting in Dublin, he advocated self-government for Ireland (4 December 1860).
Underwood's lasting fame rests on his political activities in the early 1860s as founder and president of the National Brotherhood of St Patrick. On 26 January 1861 the Irishman published a letter from him proposing that political banquets be held on St Patrick's day in Dublin, Cork, Belfast and British cities with large Irish populations. The committee formed to organise these banquets included many Fenians and gave rise to the Brotherhood of St Patrick. Underwood presided at the Dublin banquet (18 March). Within two months the brotherhood, ‘a loose collection of people with nothing in common except the vaguest attachment to the principles of nationality’ (Comerford), had 23 accredited branches. Under its auspices a committee, dominated by Fenians, organised the funeral of Terence Bellew McManus (qv). The Brotherhood of St Patrick flourished in Ireland for three years, so much so that it was repeatedly denounced by the catholic archbishop of Dublin, Paul Cullen (qv); it had much success among the Irish in Great Britain, who were to play in the 1870s a vital role in the rise of the political movement led by Charles Stewart Parnell (qv). By 1864 it was in decline in Ireland as James Stephens (qv) and other Fenians withdrew their support. In 1866 Underwood was arrested after the suspension of habeas corpus and held for 6 months. Whether he ever took the Fenian oath is doubtful, but he continued to be much trusted by Fenians.
According to D. J. O'Donoghue (qv), Underwood wrote prose and verse for the Nation and other journals. He also wrote a short drama in verse, ‘The youthful martyr’ (June 1867). It honoured a young Fenian, Richard Joseph Stowell, who had died in Naas jail; it was published in a London newspaper and later in the Irishman (4 November 1876). Some five years before his death he was appointed a trustee in a family case under the court of chancery. Later he was censured for contempt of court for not paying costs in a private suit and was incarcerated in Omagh jail. He was eventually released in poor health and two months later, on 7 October 1876, died at his home in Main St., Strabane. His funeral in Dublin (15 October) drew some 1,500 mourners and an oration beside his grave, in the same plot as McManus's, was given by the secretary of the Amnesty Association, Robert Dunne. No report has been found of any minister of religion officiating. Underwood's presbyterianism was nominal and he was anti-clerical.