McGrath, the only surviving son of Herbert Edgar McGrath (1876-1963) and Edith May Sorrell (d 1946), was born in Gladesville, New South Wales. An elder brother, Ivor, died in infancy, and his sister Eileen (who became a notable sculptor and graphic designer) was born in 1907. Herbert McGrath was born in New Zealand but his family had moved to New South Wales when he was a child and Edith Sorrell had been born in New South Wales. The couple married in 1899. Both their families were of mixed Irish and English descent.

McGrath was educated at Paramatta North Public School until 1911 when he was moved to Gladesville Public School and from there in 1916 won a high school bursary to Fort Street Boys School in nearby Sydney. In 1921 McGrath enrolled in the Faculty of Arts at Sydney University but subsequently transferred to the School of Architecture. While attending university McGrath also studied painting at the Julian Ashton School. In 1924 he published, in a limited edition of 30 copies on Japanese vellum, a book of woodcut illustrations and poetry entitled The Seven Songs of Meadow Lane. McGrath graduated in 1926 Bachelor of Architecture with first class honours and as the winner of the Wentworth travelling scholarship.

Setting up practice in London in 1930, McGrath’s first commission was to design the interiors for Broadcasting House in Portland Place, London. To assist with such a large commission, he solicited the help of Wells Coates and Serge Chermayeff; the latter was passing through London and would emigrate to America in 1940. Further interior design jobs followed, including a design for the aeroplane interiors for Imperial Airways.

McGrath’s personal major building project was the modernist circular St Ann’s Court, Chertsey in 1936. The house was built for stockbroker Gerald L. Schlesinger and his partner the landscape architect Christopher Tunnard.

McGrath was particularly interested in the architectural and decorative use of glass, writing several articles for the Architectural Review in the 1930s,and in 1937 publishing the highly influential book Glass in Architecture and Decoration. Some of his 1934 etched glass doors can still be seen at RIBA’s headquarters in Portland Place, London.

In 1940 McGrath moved to Dublin where he was appointed Senior Architect at the Office of Public Works. In 1948 he was appointed Principal Architect, a post he held until 1968. He quickly took command of the resources which were available to give a recognizable “look” to Ireland’s state buildings. These included specially-designed woolen carpets, Waterford glass chandeliers, Irish silk poplin hangings and, in terms of fittings, 18th-century chimney-pieces and ornamental plasterwork. He acted as supervisor and co-ordinator of the decor as well as the architecture, using his extensive knowledge of Irish architecture of the Georgian period.

In Dublin, McGrath’s principal concern was the restoration of Dublin Castle, which started in the late 1950s and continued for decades. Among the buildings he designed was the Royal Hibernian Academy, RHA, building in Ely Place, Dublin. He had been appointed an associate member of the RHA in 1949 and became a full member in 1967. The following year he became the Academy’s professor of Architecture. For many years, starting in 1946, McGrath championed and worked on the design for a National Concert Hall for Ireland which was to be built at Raheny. However the project was always dogged by political complications and was eventually cancelled in 1973. McGrath died in Dublin a few years later in 1977, at the age of 74.