Fergus, John (c.1700–1761), doctor of medicine and collector of books and manuscripts, was born in Co. Mayo, where a branch of the Ó Fearghusa family was to be found. His father's name was Mac Raith, and hence Dr Fergus was sometimes referred to in Irish as Seán mac Mac Raíth Ó Fearghusa; no details of his mother are known. The family, the Uí Fhearghusa, were hereditary lianna (physicians) in the Gaelic tradition, as is evidenced by poems and manuscripts written by, or addressed to, members of this family. The ‘Liber Flavus Fergusiorum’ (now RIA MS 476), an heirloom of this family, passed, probably with other manuscripts, to John Fergus.
Fergus had at least one brother, a priest. While he must have received his elementary education from hedge-school teachers and/or at home, the large number of French books, especially of a medical nature, contained in the catalogue for the sale of the combined libraries of himself and his son, Dr Macarius (Mac Raith) Fergus (d. 1763), suggests that he qualified in medicine at a French university, as many other Irish students did in that century. Unusually for a catholic at the time, he was admitted as a licentiate of the College of Physicians in 1735.
Dr Fergus practised very successfully in Dublin, becoming the most eminent catholic doctor in the city in his time. He lived for a period in Jervis St., and later in Cavendish Row. He gave his services free of charge on two afternoons in the week to the Charitable Infirmary (later Jervis St. Hospital). Fergus seems to have had a fine library. He had a great interest in Irish culture, and in 1731 acquired some of the manuscripts of John Conry (1704–69). He was a noted patron of scribes and is celebrated by Tadhg Ó Neachtain (qv) in his well known poem ‘Sloinfead Scothadh na Gaoidhilge grinn’ (c.1727), which devotes a verse each to twenty-six Irish scholars, scribes, and patrons, active in Dublin at that time. It is known that Dr Fergus gave patronage to Aodh Ó Maolmhuaidh, Muiris Ó Gormáin (qv), and Mícheál Ó Longáin (qv), well known scribes. Carolan (qv), the noted harper, composed a piece in his honour.
John Fergus became friendly with Charles O'Conor (qv) of Belanagare (Bellanagare, Co. Roscommon) in the late 1720s and the two became firm friends and correspondents, seeking and exchanging books and manuscripts. When O'Conor published his Dissertations (1753), Dr Fergus claimed that O'Conor had in this published as his own an idea of Fergus's, and though O'Conor strenuously denied this, the friendship did not survive. (Fergus may have been planning to write a historical book on Ireland himself.)
The grant in Fergus's estate has been dated 19 February 1761, suggesting that he died very late in 1760; however, Wilson's Almanac for 1761, the last in which he appears, lists him as being in practice with his son in Cavendish Row, and Faulkner's Dublin Journal in the issue of 21–24 March 1761 reports his death (which seems to have been either 14 or 21 March). He married (date unknown) Mary French; apart from their son, at least two daughters were born of the marriage.
A catalogue was printed for the sale of the Fergus combined libraries, which took place on 3 February 1766. It contains 2,479 numbered items and constitutes a fine collection, with many medical items and Latin and Greek texts, as well as many books of Irish interest. The original is now in the Henry E. Huntington Library, San Marino, California; a copy in the Gilbert Library, Dublin, is reproduced from a microfilm. The catalogue includes thirteen printed books in Irish and thirty-six Irish manuscripts, some of them very important, including the Annals of Ulster, the Annals of Loch Cé, an important genealogical manuscript by Dubhaltach Óg Mac Firbhisigh (qv), and one of the original volumes of the Annals of the Four Masters. Substantial purchases were made at the auction on behalf of the library of TCD, Lord Drogheda, Lord Jocelyn (Robert Jocelyn (qv), later 1st Earl Roden), Charles O'Conor of Belanagare, Muiris Ó Gormáin, and others. A miniature of the doctor was still in the possession of his descendants in 1853.