His superiority as a classical scholar led to his appointment as Professor of Humanity in the University; but being ardent in upholding the Presbyterian policy, he became obnoxious to the episcopal party, and so was compelled to relinquish his office.
On the recommendation of Robert Blair, then a regent of Glasgow College, he proceeded to the north of Ireland, where a colony from the west of Scotland had been planted.
For a time he officiated at Oldstone, and having been ordained by Andrew Knox, Bishop of Raphoe, who is said to have regarded him as a relative , in 1826 he settled at Templepatrick, county Antrim, as chaplain to Captain Norton.
"Here lies interred under this stone,Great Knoxes grand child, John Welshes son ;Born in Scotland, and bred up in France,He then came to Ireland the gospel to advance."
Appended to a declaration for settling the Province of Ulster, dated Carrickfergus, 23 May 1653, are the names of 260 persons, in the counties of Down and Antrim, whom Cromwell's commissioners proposed to remove to certain districts in Munster.
The Six-Mile Water district included the parish of Templepatrick, and those enumerated in "the Declaration" were persons obnoxious to Cromwell on account of their adhesion to monarchical and Presbyterian principles.
Josias Welsh was minister at Templepatrick in the north of Ireland, commonly called the Cock of the Conscience by the people of the country, because of his extraordinary wakening and rousing gift.