Ferguson, Lady Mary Catherine (1823–1905), antiquarian and biographer, was born 13 September 1823 in Stillorgan, Co. Dublin, eldest child and only daughter of Robert Rundell Guinness (1789–1857), founder of the banking firm of Guinness Mahon, and Mary Anne Guinness (née Seymour; d. 1837). Educated at home and at Woodside School in Cheshire, from an early age she showed an interest in Irish archaeology. She met her future husband, Samuel Ferguson (qv), then a young barrister, at a dinner party in Dublin, where she made an immediate impression on him during an animated discussion on medieval Ireland. They married after a relatively short courtship (August 1848), and after a period living in Howth, Co. Dublin, settled permanently in North Great George's St., Dublin. Though they had no family of their own, for several years they reared the children of her brother-in-law John Ferguson after the death of their mother. The Ferguson home became a meeting-place for leading Irish scholars, writers, and artists; however, critics – noting her ascendancy tory background – have suggested that through her contacts she and her husband also came to cultivate the more staid company of anglican clergymen and reputable physicians.
Closely involved in her husband's researches, she regularly accompanied him on his archaeological excursions throughout Ireland, and after his death (1886) much of her time was spent promoting his memory and reputation. She wrote his biography, Sir Samuel Ferguson in the Ireland of his day (1896), and prepared numerous popular editions of his work, among them Ogham inscriptions in Ireland, Wales and Scotland (1887), Lays of the western Gael (1887), Congal (1893), and Lays of the Red Branch (1897). She also produced an historical work of her own, Ireland before the conquest (1868, reprinted 1890), and published a biography of her friend the antiquarian William Reeves (qv) in 1893. In later years she became associated with the campaign for women's admission to higher education. She was closely associated with Alexandra College, Dublin, taking charge of the College Literary Society's class on Shakespeare. She died at her Dublin home 5 March 1905, and was buried alongside her husband at Donegore, Co. Antrim.