He was born at the rectory, Kilsallaghan, County Dublin, of which parish his father, John Gregg, was then rector, on 3 May 1834. His mother was Elizabeth Law. In 1862, he returned to the Diocese of Cork as rector of Frankfield and chaplain to his father, then Bishop of Cork.
He was educated at Trinity College, Dublin, where he graduated Bachelor of Arts with honours in 1857, and proceeded Master of Arts
In 1860. In the same year Gregg was ordained for the curacy of Rathcooney, County Cork, and three years later was appointed rector of Christ Church, Belfast, an important cure which brought him in touch with the working-class population of the north of Ireland. In 1865 he became rector of Carrigrohane and precentor of Saint Finance, Financial Barre"s Cathedral, Cork. Here he quickly acquired a high reputation for administrative ability, as well as for the qualities of sound judgment, moderation, and good sense by which he was subsequently distinguished in the episcopal office.
In the controversies which followed, the disestablishment of the Irish church, particularly in regard to the revision of the prayer-book, Gregg took the conservative side, but his influence was uniformly exerted in a conciliatory spirit.
In 1874, Gregg was appointed Dean of Cork, and in the following year was selected by the Irish bishops to succeed James Thomas O"Brien in the diocese of Bishop of Ossory. On his father"s death on 26 May 1878, the synods of Cork, Cloyne and Ross at once selected Gregg to succeed him.
On the death in 1893 of Primate Robert Bent Knox, Gregg was selected to succeed him as Archbishop of Armagh and Primate of all Ireland.
As Bishop of Cork, Gregg"s most noticeable work lay in the completion of the beautiful cathedral of Saint Finn Barre, which had been rebuilt during his father"s episcopate at a cost of over £100,000. But he also won a deserved reputation not only for administrative efficiency, but for a statesmanlike grasp of church problems which opened the way to the highest office in the Irish church.
Gregg, at forty-one years of age, thus became a member of the episcopal bench while his father was still Bishop of Cork, Cloyne and Ross.
