Two of his classmates ordained on the same day would also serve as Irish bishops, William Cardinal Conway and Patrick Mulligan.
All three entered Maynooth's Dunboyne Institute, and in 1939 McDonnell was awarded a Doctorate of Divinity on the topic of Mass Stipends.
He was appointed Administrator of the Diocesan Cathedral in 1961, and two years later Parish Priest of Dromore West in County Sligo.
[citation needed] McDonnell was appointed Bishop of Killala on 12 October 1970, and received episcopal ordination from the hands of his erstwhile classmate William Cardinal Conway on 13 December 1970.
[3] Shortly after his retirement, a meeting of the clergy of Killala was convened at which a ten-point plan was issued outlining their hopes for pastoral changes in the diocese.