McGrath was born on 23 January 1953 in Belfast, Northern Ireland, and grew up in Downpatrick, County Down, where he attended Down High School. In September 1966 he became a pupil at the Methodist College Belfast, where his studies focused on mathematics, physics and chemistry. He went up to Wadham College, Oxford, in 1971 and gained first-class honours in chemistry in 1975. He began research in molecular biophysics in the Oxford University Department of Biochemistry under the supervision of George Radda[24] and was elected to an E.P.A. Cephalosporin Research Studentship at Linacre College, Oxford, for the academic year 1975–1976, and to a Domus Senior Scholarship at Merton College, Oxford, for the period 1976–1978. During these three years, he carried out scientific research while studying for the Oxford University Final Honour School of Theology. He was awarded an Oxford Doctor of Philosophy degree for his research in molecular biophysics (December 1977), and gained first-class honours in theology in June 1978.
