Hayden, Thomas (1823–81), physician, was born into a landowning family in Parsonshill, near Fethard, Co. Tipperary. His father's cousin was Bartholomew Hayden (1792–1857), born in Co. Tipperary, who had a distinguished career in the Brazilian navy, reaching the rank of commodore. The Haydens were protestant, but Thomas's father (whose first name is unknown), had married a Roman catholic, Margaret, daughter of Thomas Crean (or Crane) of Bushy Park, Co. Tipperary, and their children were brought up as catholics.
Educated at Tramore College, Co. Waterford, Thomas then studied medicine at ‘The Original School’ in Peter St., Dublin, which had been founded by a group of doctors including Thomas's older cousin, George T. Hayden – a colourful character, and a doctor/apothecary who had frequent disagreements with the then evolving ‘medical establishment’. The younger Hayden also studied at the Meath Hospital, Dublin; in 1850 he received his surgical licence from the RCSI, and then taught anatomy at ‘The Original School’. He was appointed surgeon to the Anglesea Lying-in Hospital (also in Peter St.), and in 1852 he received his fellowship of the RCSI. (The medical establishments in Peter St. later evolved into the Adelaide Hospital.) In 1860 Hayden became a licentiate of the RCPI; he was elected fellow in 1867, and in 1875 served as vice-president.
Thomas was much involved with John Henry Newman (qv) in establishing the system for medical education in the context of the new Catholic University (opened in 1854), in which he became the first professor of anatomy. (Archbishop Paul Cullen (qv) had mistakenly believed that Hayden was protestant, and had in a letter of 31 July 1855 warned Newman against appointing him.) The medical school in Cecilia St. was the most successful element of the Catholic University, and remained so even after Newman returned to England in 1857. Hayden was also a contributor to Newman's magazine Atlantis. He was physician to the Mater Misericordiae Hospital from its foundation (1862) until his death, and also had a private practice in Merrion Square. He was so mild-mannered and courteous that he was nicknamed ‘Gentle Thomas’.
Hayden contributed papers on anatomy, physiology, and pathology to medical journals, and his major book Diseases of the heart and aorta (1875) was regarded as an important contribution in its day. He was elected a member of the RIA (1857), and was later its vice-president. A senator of the RUI, he developed a cold after attending a senate meeting in October 1881, and died three weeks later (29 October 1881) of pneumonia; he was buried in Glasnevin cemetery beside his wife.
He married (1857; probably in September), at Thurles, Co. Tipperary, Marianne (d. 1871), daughter of Patrick (or Thomas) Ryan, landowner, of Rathfanna or Rathfalla, Thurles. They had one son, John (1859–1936), who became a barrister in England and the US, and wrote poetry and a novel; and one daughter, Mary Hayden (qv), a noted historian and activist in language, national, and women's issues. A bust of Thomas Hayden is in the RCPI, and his photograph features in an album of college fellows, held in the college archives.