Hello trinity2,
I looked at the online extracts of Griffith's Valuations of Donegal which took place in the county in 1857 for the surname McDonagh. If you try to search names starting with Mc you tend to get a nil result so it's best to drop the Mc and search for Donagh. You'll find that Mc comes at the end of the list. There were a larger number of McDonaghs in Derry than in Donegal. I feel it would probably be a better tactic to search for Bridget's family than John's family as marriages tended to take place in the bride's parish.
Thomas Mc Donagh, Fanaghans, Inver
Patrick Mc Donagh, Market Square, Carndonagh, Donagh
Bernard Mc Donagh. Glenkeeran, Raphoe
Patrick Mc Donagh, Glencrow, Moville Upper
Patrick Mc Donagh, Malin Road, Moville, Moville Lower
James Mc Donagh, Pound Street, Carndonagh, Donagh
Catherine Mc Donagh, Garrowcarry, Kilmacrenan
Mary Mc Donagh, Fair Green Row, Moville, Moville Lower
Michael Mc Donagh, Ballybrollaghan, Inver
Good luck with the search.
Chris