Byres was a member of a family of Scottish Jacobite sympathisers who settled in Rome in 1758, where he became a cicerone and an art dealer, mainly to Scottish and English gentlemen on the Grand Tour until his return to Scotland in 1790. His house was in Via Paolina.
Byres was a painter and an adept designer, whose Vanvitellian design for a palazzo facade won a prize from the Accademia di San Luca in 1762. In Rome members of his circle were drawn by Angelica Kauffman in a sketchbook she used from 1762 to 1764: the portraits include the English painter Nathaniel Dance, Gavin Hamilton, and the abbé Peter Grant. By 1764 he was so well acquainted with the ancient sites and the cabinets of collectors that he took about a party of colonial Americans, including Samuel Powel of Philadelphia, who unlike his British peers, took assiduous notes. Byres, as well as some others British residents in Rome such as Thomas Jenkins and Colin Morison, worked as an art dealer, working with important European collectors.
William Constable purchased from Byres many of the Italian paintings and marble copies after Roman sculptures at Burton Constable, Yorkshire, and Byres was responsible for introducing the artist Anton Maron, who painted William Constable and his sister in the pose and dress of Cato and Marcia. Among the antiquities that passed through his hands, the most famous may be the Portland Vase, which he sold to Sir William Hamilton in 1770. Among the commissions for which he acted as agent was the Noli me Tangere of Raphael Mengs, 1771, for an altarpiece for All Souls College, 1771.
A clear idea of his own collection can be gleaned from a 1790 inventory made upon his return to Tonley. Though he sent many of his clients to Pompeo Batoni, the only Batoni portrait hanging in his house was of his sister Isabella, Mrs Robert Sandilands.
Concerning the Etruscans Byres formulated the hypothesis that Etruscan literature has not come down to us because it was purposely destroyed by the Romans.
Before he left Rome in 1790 he made a payment to the maître d'hôtel of Henry Benedict Stuart, Cardinal York in favour of the Duchess of Albany, illegitimate daughter of Bonnie Prince Charlie, so it may be inferred that his Jacobite sensibility ran deep.
- His father Patrick Byres went abroad after Battle of Culloden; "The Byres Family: An Eighteenth Century Portrait Group", The Burlington Magazine for Connoisseurs 82 No. 479 (February 1943, pp. 46-47, 49, p 48.
- Waterston, Charles D; Macmillan Shearer, A (July 2006). Former Fellows of the Royal Society of Edinburgh 1783-2002: Biographical Index (PDF). I. Edinburgh: The Royal Society of Edinburgh. ISBN 978-0-902198-84-5. Archived from the original (PDF) on 4 October 2006. Retrieved 19 November 2011.
- John Fleming, in Connoisseur Year Book 1959 pp 24-27 and Fleming Robert Adam and His Circle (Harvard University Press) 1962, pp 306, 378.
- Peter Grant of Blairfindry, head of the Scottish Mission. Arthur S. Marks, "Angelica Kauffman and Some Americans on the Grand Tour" American Art Journal 12.2 (Spring 1980, pp. 4-24) p. 5.
- Samuel Powel's "Short Notes on a Course of Antiquities at Rome in Company with Messers Apthorp Morgan & Palmer begun May 21, 1764 under Mr Byers Antiquarian" is conserved at the American Philosophical Society, Philadelphia; (Marks 1980:11 note 31).
- Christopher Gilbert, "William Constable as Patron (1721-1791) at Hull" The Burlington Magazine 112 No. 804 (March 1970), pp. 178, 183-184).
- John Sparrow, "An Oxford altar-piece" The Burlington Magazine 102 No. 682 (January 1960), pp 2, 4-9.
- Francis Russell, "Batoni's Mrs Sandilands and Other Portraits from the Collection of James Byres" The Burlington Magazine 120 No. 899 (February 1978), pp. 114, 116-117.
- P. Coen, Il mercato dei quadri a Roma nel XVIII secolo: , Florence, Leo S. Olschki, 2010, pp. 70-77
- I. Bignamini, C. Hornsby, Digging And Dealing In Eighteenth-Century Rome (2010), p. 246-248
- P. Coen, La carriera di mercante d'arte e il profilo culturale di James Byres of Tonley (1737-1824), in La città degli artisti nell’età di Pio VI, a cura di L. Barroero, S. Susinno, «Roma moderna e contemporanea», X, 2002, pp. 153–178
- Brinsley Ford, 'James Byres, principal antiquarian to the English visitors to Rome', in Apollo; 99 (June 1974), pp 446–61.
- W.T. Whitley, Artists and Their Friends in England 1700-1790 (1928) ii, pp247–48.