O'Halloran, Sylvester (1728–1807), surgeon and historian, was born 31 December 1728 at Caherdavin, Co. Clare, third son of Michael O'Halloran, a prosperous catholic farmer, and Mary O'Halloran (née McDonnell). His brothers were Joseph Ignatius, later a member of the Society of Jesus, and George, a silversmith and man of property in Limerick. Educated by his mother's kinsman Seán Claragh McDonnell, a teacher accomplished in Greek, Latin, and Irish, and at a school in Limerick run by Robert Cashin, a protestant clergyman (with Peter Woulfe (qv) as fellow-pupil), he went abroad when 17 or 18 to study surgery in London, Paris, and possibly Leyden. With an enquiring mind and a bent for research, he attempted to elucidate the cause of cataract. He observed the effects of immersing the eyes of freshly killed calves in a variety of solutions, and experimented, too, on living dogs to study the blood-supply of the eye.
O'Halloran appears to have returned to Ireland by early 1749, establishing a practice in Limerick as a surgeon and practitioner of midwifery. He created a stir, his tall slender figure attired in French costume, and with an impressive wig, cocked hat, and gold-headed cane. The publication of his Treatise on the glaucoma or cataract (1750) left him free to study the properties of atmospheric air, then a challenging subject. It must have mystified the populace to see him ascending the steeple of St Mary's church with air-filled bladders, repeating his experiments on a nearby hill. The manuscript of the unpublished ‘A new philosophical & medical treatise on the air’ is held by the library of the RIA, of which he was a member. He was one of the founders of the Co. Limerick infirmary, opened in 1761.
His mind was much occupied by the surgical problems of the day. A new method of amputation appeared in 1763; his book on Gangrene and sphacelus came out in 1765; his most mature surgical work, A new treatise on external injuries of the head, delayed for want of a publisher, was not ready until 1793. He had become an admirably conservative authority on the treatment of head injuries. Cerebral compression required surgical intervention, which should be avoided in concussion. The horse occupied the role as a traumatic agent latterly taken by motor vehicles. Violence was commonplace, and aggravated by alcohol. He might have four broken skulls to treat on a May morning.
O'Halloran's ‘Proposals for the advancement of surgery in Ireland’ took the form of an appendix to Gangrene. An admirable educational blueprint, strongly influenced by the Académie Royale de Chirurgie which he had seen in Paris, it advocated in summary the creation of a suitable building in the capital, the appointment of professors, and the listing of competent surgeons. This document is thought to have played a significant part in the foundation of the RCSI, to which he was elected honorary fellow in 1786.
O'Halloran participated actively in the social life of Limerick city. He was president of the Free Debating Society in 1772, and in 1783 was elected to the Citizens of Limerick Committee set up to enquire into the state of the Shannon navigation. His principal avocation was history: he published An introduction to the study of the antiquities of Ireland (1770), Ierne defended (1774), and A general history of Ireland (1775). While generally well received, these books were not accorded universal acclaim. A critic advised the doctor to discontinue scribbling, and to mind the functions of the human body, which he probably understood better than he understood history. He was, however, included by Gilborne in The Medical Review: ‘In Limerick O'Halloran resides, / And o'er the county hospital presides; / Excels in surgery and healing arts, / With flowing pen displays uncommon parts . . . ’. A prolific letter-writer, O'Halloran corresponded with Edmund Burke (qv), Charles Vallancey (qv), Charles Lucas (qv), and Charles O'Conor (qv) of Belanagare, among others. Four of his letters to O'Conor are devoted largely to a discussion of Macpherson's verses. He offered his copy of Mícheál Ó Cléirigh's (qv) Foclóir nó sanasain nua (‘Irish vocabulary’) to O'Conor, and wished to borrow from him an Irish translation of Hippocrates. The preface to Reliques of Irish poetry (1789) by Charlotte Brooke (qv) acknowledged her ‘innumerable obligations’ to Sylvester O'Halloran.
As the century ended he was strongly opposed to the act of union. His name – ‘O'Halloran, MRIA’ – appeared in the Limerick Chronicle for 15 January 1800, in a list of signatures resisting the proposed union. After a stroke O'Halloran was infirm and confined to his chair for some time before his death at his home on Limerick's Merchant Quay, 11 August 1807. His remains were laid in the family vault in Kileely cemetery.
He married (1752) Mary Casey (d. 1782) of Ballycasey, Co. Limerick; they had four sons and a daughter and lived in Change Lane, moving later to Merchants' Quay. At least two of his grandsons settled in Australia: Thomas Shuldham O'Halloran (1797–1870) and William Littlejohn O'Halloran (1806–85), sons of Sir Joseph O'Halloran (1763–1843), the surgeon's youngest son, who, having given 53 years' uninterrupted military service in India, was made an honorary MRIA and freeman of the city of Limerick.