(c. 1792–1848). Irish architect who designed and built the first scholarly Gothic Revival buildings in that country. He practised from Newry, Co. Down, and formed a partnership with Thomas Jackson of Belfast. His masterpiece is the RC Pro-Cathedral of St Patrick, Dundalk, Co. Louth (1835–47), in the English Perpendicular style. In 1838 he prepared designs for St Patrick's Cathedral, Armagh (also Perpendicular), but McCarthy completed the building in the Second Pointed style, leaving Duff's incongruous Perpendicular nave-piers supporting stylistically earlier elements. With Jackson, he designed the Greek Revival Old Museum, College Square North, Belfast (1830–1). He was also responsible for a number of Classical (e.g. extensions to Hillsborough Castle, Co. Down (c. 1830–40)) and Tudorbethan (e.g. Parkanaur, Castlecaulfeild, Co. Tyrone (1839–48); Narrow Water Castle, Warrenpoint, Co. Down (c. 1831–7)) country-houses, as well as court- and market-houses, the latter usually in a sober Classical style (e.g. Hilltown (1828) and Newry (1841), Co. Down). His assistant was W. J. Barre.
From A Dictionary of Architecture and Landscape Architecture in Oxford Reference.