Dillon, Thomas Patrick (1884–1971), nationalist and chemist, was born 15 January 1884 in Enniscrone, Co. Sligo, first child among four sons and a daughter of John Dillon, engineer, and his wife Elizabeth, daughter of William Kirby Sullivan (qv). His father was a nephew of John Blake Dillon (qv), and other catholic politicians and lawyers were among his relatives. The family moved to Ballina, Co. Mayo, where John Dillon built the town waterworks. Thomas was educated at the local diocesan college, St Nathy's, Ballaghadereen, and at Clongowes Wood College. At the age of sixteen he won a scholarship to study medicine at QCC. After the mandatory preliminary course in arts, he decided to take a BA in chemistry and physics (1904) instead of a medical degree, and proceeded to the Royal College of Science, Dublin, where he was awarded an MA in chemistry (1908). Hugh Ryan (qv), then professor of chemistry at the Catholic University school of medicine, appointed him as his assistant at a salary that averaged £6 per month. He supplemented his rather meagre earnings by teaching science at the Catholic University School and Loreto Convent School, Dalkey. In 1908, after the establishment of the NUI, he transferred along with Ryan to UCD, where in 1912 he was awarded the first D.Sc. degree conferred by the NUI.
Through the influence of Tom Kettle (qv) and Joseph Plunkett (qv) he became involved in republican and labour politics, holding the position of honorary secretary of the Industrial Peace Committee during the lock-out of 1913 and later acting as chemical adviser to Plunkett's associates in the Irish Volunteers, helping to manufacture hand grenades and bombs. One of his students in the chemistry class in UCD was Joseph Plunkett's sister Geraldine Plunkett (1891–1986), daughter of Count George Noble Plunkett (qv), who strongly supported radical republicanism. Thomas Dillon and Geraldine Plunkett married in Rathmines on 23 April 1916, which was Easter Sunday. The morning after the wedding, from the Imperial Hotel, O'Connell St., they watched the early events of the Easter rising, centred on the GPO across the street. Dillon, though not aware of detailed plans for the rebellion, had been asked to take charge of chemical works once they had been commandeered. When this failed to happen, he was told to await orders at home. The couple were able to escape from the city centre, though their luggage was destroyed when the hotel was shelled shortly afterwards. Dillon was not arrested, and assisted his father-in-law to organise a meeting in Dublin of delegates from nationalist groups from all over the country. He refused to become secretary at the October 1917 Sinn Féin convention but agreed to serve on the executive council. This led to his arrest in May 1918 and his internment for almost a year in Gloucester prison in England, where he learned Irish.
He applied for the post of professor of chemistry in UCG while still in jail, and was appointed on his release in March 1919, although there was bitter opposition on account of his involvement in the IRB and Sinn Féin. He and his wife took active part in the war of independence; he sat as a judge in the Sinn Féin courts in 1920, and in February 1921 he organised a raid on a courthouse, in search of records. For months at a time, evading arrest, Dillon seldom slept at home. Geraldine Dillon was briefly jailed, and their house was searched, roughly, by the Black and Tans.
Despite such distractions and difficulties, he eventually achieved an international reputation for his work to elucidate the structures of carbohydrates, particularly those of which seaweeds are composed. His pioneering research on waxes, alginic acid, carrageen, laminarin and other alginates and gums was published from 1928 in journals such as the Scientific Proceedings of the Royal Dublin Society, Journal of the Chemical Society, Chemistry and Industry, and the Proceedings of the Royal Irish Academy. In 1939 he took out two patents on techniques using seaweed in the manufacture of paper and of fertiliser. The department of chemistry in Galway in Dillon's time developed a strong research focus on the analysis and synthesis of carbohydrates. A state-sponsored unit was set up within the university, employing Patrick Moynihan and others to explore the economic uses of seaweed – one of the few natural resources available for the possible development of a chemical industry in the region round Galway. Although Dillon was not a native speaker of Irish, he encouraged the teaching of chemistry in the Irish language, and in the early 1940s all the first-year and parts of the second- and third-year courses were taught through Irish, the lectures being given by Vincent Barry (qv) and Prionsias Ó Colla. With Barry he wrote the first chemistry textbook in Irish. Under Dillon's guidance the department flourished and student numbers rose from seventy in 1919 to 300 in 1953. He helped set up the structures within which chemistry in Ireland developed as a profession, and was a founder member of the Chemical Association of Ireland (1922–36), and the Irish Chemical Association (1936–50). He served as president of the Institute of Chemistry of Ireland (1954–6). He was a member of the RIA from 1941, and its vice-president (1957). In 1954 he received an honorary doctorate from the University of Dublin.
Dillon retired in 1954; he and his wife had lived apart for some time, but eventually went to live with their daughter in Dublin. Dillon died 11 December 1971, survived by his wife and three daughters and two sons, including Eilís Dillon (qv) (1920–94), novelist, and Michael Dillon (qv) (1922–92), an agricultural journalist. Another son had died aged three. Geraldine Plunkett Dillon's memoirs (published as All in the blood (2006)) preserve a vivid and valuable record of a remarkable family's remarkable influence on the foundation of modern Ireland. She died 13 May 1986 in Dublin.