John Joseph Therry, a Catholic priest, was the son of John Therry, of Cork, Ireland, and his wife Eliza, née Connolly. He was educated privately and at St Patrick's College, Carlow. Therry's interest in Australia, aroused by the transportation of Irish convicts and the publicity surrounding the forced return of Father Jeremiah O'Flynn in 1818, eventually resulted in his sailing from Cork under a senior priest, Father Philip Conolly, in the Janus, which carried more than a hundred prisoners. They arrived in Sydney in May 1820.
In 1821 Father Conolly went to Van Diemen's Land [later Tasmania], leaving Therry for five years as the only priest on the mainland. Articulate and thorough, he attended to every aspect of the moral and religious life of the Catholics. He travelled unceasingly, living with his scattered people wherever they were to be found. His influence was also impressive among the Protestant settlers and the convicts. Therry described his life in Australia for forty-four years as 'one of incessant labour very often accompanied by painful anxiety'.