Gregg, Robert Samuel (1834–96), Church of Ireland primate, was born 3 May 1834 at Kilsallaghan, Co. Dublin, second son among three sons and three daughters of John Gregg (qv), bishop of Cork, and Elizabeth Nicola Gregg (née Law). Entering TCD, he graduated BA (1857), MA (1860), BD and DD (1873) (Dubl.). Following ordination into the Church of Ireland (1857) he was successively appointed curate of Rathcooney, Cork (1857–9), and incumbent of Christ Church, Belfast (1859–62). He transferred to the Cork diocese on his father's election as bishop of Cork, becoming rector of Frankfield (1862–5), rector of Carrigrohane and precentor of St Fin Barre's cathedral, Cork (1865–74) and dean (1874–5).
As hon. secretary (1871–5) and twice president of the general synod, he excelled as an administrator and financier. In the years preceding the disestablishment of the Church of Ireland, he had assisted in the framing of a new constitution for the church. During the controversies following disestablishment, he was conservative and conciliatory; his administration of diocesan finances in Cork became a model for the post-established church and he published a pamphlet, Church finance (1871). An evangelical like his father, he joined him on the episcopal bench as bishop of Ossory, Ferns, and Leighlin (1875); his father participated in his consecration, and on his death Gregg succeeded him as bishop of Cork, Cloyne, and Ross (1878). He presided over the completion of the remarkably lavish (and expensive) neo-Gothic St Fin Barre's cathedral (begun by his father), laying the top-stone on the central spire in 1879. In the same year he published Memorials of the life of John Gregg, D.D. In 1893 he was elected archbishop of Armagh and primate of all Ireland.
Dignified and kindly, he was a popular and respected figure. He died 10 January 1896 at the Palace, Armagh, and was buried beside his wife at Frankfield, Co. Cork. A portrait by R. P. Staples (1853–1943) hangs in the Palace and a memorial window was placed in the cathedral in Armagh; in Cork there is a memorial stone in St Fin Barre's cathedral and a portrait adorns the Palace.
He married (1863) Elinor Bainbridge of Frankfield; they had a son and a daughter. His nephew J. A. F. Gregg (qv) also became primate of all Ireland (1939–59).