He entered the priesthood of the Catholic Church, and was consecrated Bishop of Raphoe, whose diocese covered most of County Donegal, in November 1725 at Drogheda.
In May 1737, Gallagher was translated from the bishopric of Raphoe to that of Kildare,[1] and in the same year he was appointed administrator of the Diocese of Leighlin.
[1] This work, with Gallagher's certificate prefixed, was printed in the following year at Paris by James Guerin.
In his preface the author mentioned that he had composed those discourses principally for the use of his fellow labourers, to be preached to their respective flocks, as his repeated troubles debarred him "of the comfort of delivering them in person".
He added: I have made them in an easy and familiar style, and of purpose omitted cramp expressions which be obscure to both the preacher and hearer.