John Cantwell was born in Rahan, near Tullamore, County Offaly on Christmas Day 1792, to Edward Cantwell (1750–1831) and his wife Catherine Flynn (1760–1842).
He went to Maynooth College to study for the priesthood at a very early age.
He was appointed Dean of the Ecclesiastical College, before being ordained a priest ages 22 in 1815.
[1] He served as parish priest of Kilbeggan, County Westmeath, and in 1830 he was called upon to become Bishop of Meath.
Bishop Cantwell encouraged the building of schools and churches in the diocese, and the presences of religious orders.