O'Gorman, Thomas (‘The Chevalier’) (1732–1809), officer in French service, scholar, and entrepreneur, was by his own account born 16 September 1732 at Castletown, Co. Clare, son of Matthew O'Gorman (or MacGorman) and his wife Margaret, daughter of Donough O'Loughlin of Castletown, prince of the Burren. O'Gorman had a brother and a sister; each later had a son for whom he was able to secure appointment in the Spanish forces. He probably attended a hedge school in Clare, before being educated at Paris in the Irish College and later at the Collège Mazarin, where he qualified as a medical doctor. This indicates that he knew Latin and Greek; as a fluent Irish scholar, probably brought up in the Burren law-school tradition, he was able to write an essay explaining and codifying ancient Irish law. It is doubtful whether he ever practised medicine; there were few opportunities in Paris for a young Irish doctor. He joined Walshe's regiment – in which his father had died (1740) – of the Irish Brigade, and later volunteered for service in the American war of independence. While still a student he eloped with and married (1757) the daughter of the comte d'Éon, of a noblesse de robe family owning extensive vineyards at Tonnerre, Burgundy. Her parents, who at first disapproved and attempted to disinherit her, were eventually reconciled and may even have used influence to obtain the title ‘Chevalier’ for O'Gorman. The estate remained in the possession of his brother-in-law Charles Geneviève d'Éon de Beaumont, chevalier d'Éon, the well known diplomatic agent and female impersonator. During d'Éon's many absences on state business, O'Gorman took part in marketing Tonnerre wines. Tirelessly extolling the excellence of its burgundy and champagne, he sent large quantities as samples to potential customers – notably Benjamin Franklin, whose help O'Gorman hoped for in gaining land in America (1773) or the post of French consul in Philadelphia (1778). As late as 1793 the Chevalier, though no longer resident in France, was selling wine to the chief baron and to Lord Inchiquin, but no later connection with Tonnerre can be found.
His main income before the revolution came from producing pedigrees for Irish expatriates to secure them social acceptance and advancement in the service of France, Spain, and Austria. His fees ranged from twenty guineas (£21) for a de Lacy pedigree to £400 for the Kearneys of Co. Clare, £1,000 for the O'Reillys, £1,600 for the McCarthys of Toulouse, and £1,800 for the Comte de Butler. Not all transactions went smoothly and profitably: in 1788 he was in dispute with Lord Dunsany over the custody of Plunkett family papers and payment of fees. While engaged in genealogical research, O'Gorman became involved in the current cultural revival in Ireland. The Dublin Society authorised him to apply to the College of the Lombards in Paris and other learned bodies for copies of manuscripts removed from Ireland during the previous hundred years. By 1772 he had become involved with the short-lived committee of antiquaries formed by members of the Dublin Society, including Sir Lucius O'Brien (qv), Thomas Leland (qv), librarian of TCD, who allowed him to use the library, and Charles Vallancey (qv), to whom he supplied much of the phonetical and grammatical material for his Grammar. Through O'Gorman's initiative the Book of Lecan, then in the Irish College, was examined, copied, and subsequently presented to the Academy. Through his connections with exiled Gaelic aristocracy he was able to track down ‘Brian Boru's harp’, and with the financial assistance of William Burton Conyngham (qv) – descendant of the family that received the confiscated estate of O'Gorman's grandfather – give it to TCD. Having learned from Muiris Gorman to decipher ogham script, he was able to translate (1785) the Book of Ballymote at Auxerre, where he permitted Beranger (qv) to copy some of its illustrations before presenting it to the RIA. He is said to have procured the works of Hugh MacCurtin (qv) during his affluent years and brought them to France, where they were probably destroyed during the revolution or sold to pay some of his debts.
Debt became an increasing problem. In 1797 he wrote to the chief secretary, Thomas Pelham (qv), complaining that his little house in Dublin had been broken into by bailiffs, and that he had been arrested for a debt of £30 and accused of communicating with the French. Pelham sent a generous reply, a gift of £10, and a warrant for his release. Shortly afterwards O'Gorman returned to Clare, staying with successive relatives till his O'Brien cousins in Drumellihy let him use a dilapidated cottage, where he died 18 November 1809. Wanting to be buried with his family in the little churchyard at Coad near Killinaboy, he had had his name carved on a gravestone (since removed or destroyed in over-zealous clearing of the graveyard). However, a heavy snowfall on the day of his death made the roads too dangerous for the journey to Killinaboy, and he was buried in Kilmacduane. The Limerick Chronicle paid tribute to the social distinction, grandeur, and influence attained in France by one who ‘died in an obscure spot on the margin of the Atlantic Ocean’.
O'Gorman rarely lived with his first wife, who died (1792) after years of neglect and ill treatment, but they had three sons. The eldest (Count Joseph O'Gorman d'Éon) and second (Louis) served as officers in the French navy; the youngest was in the army. After the revolution all three transferred to English regiments. O'Gorman later married Mary, the wealthy widow of Denny Baker Cuffe of Cuffsborough, Queen's Co. (Laois). They had no children.