Here are possible marriages for your g grandparents' starting with one marriage I found in Donaghadee.
1) 26 Apt 1872, James Melville married Mary Kennedy in the Millisle Presbyterian Church in the Parish of Donaghadee, Newtownards, Co Down
2) 12 Nov 1883, James Melville married Mary Gordon in Newtownards Civil Registrars Office, in the Parish of Newtownards, Newtownards, Co Down
3) 19 May 1874, the couple with same names above, married in Carrickfergus Congregational Church, in the Parish of Carrickfergus, Larne, Co Antrim
4) 25 Dec 1882, James Melville married Robertina Maxwell in Saint Enochs Presbyterian Church, in the Parish of Shankill, Belfast, Co Antrim
5) 22 Aug 1857, James Melville & Mary Hughes married in Newry Civil Registrars Office, in the Parish of Newry, Newry, Co Armagh/Down
6) 20 Sept 1848, James Melville married Margaret Bailey in Ballyhemlin Non-Subscribing Presbyterian Church in the Parish of Ballyhalbert, Newtownards, Co Down.
I found some birth registrations in Donaghadee with the parents named: James Melville & Mary Ann McCollum and James Melville & Margaret Baille (Bailey)
This got me thinking it is possible that your John's birth registration was under misspelt surname--here is the possible surnames...
Melly, Mellon, Melvin, OMelvena--the first two I doubt but last two are likely possibles.
I think it would be a worthwhile to find out which is the nearest CoI in Donaghadee to see if they have your grandad's birth/baptism entry in their parish records?
Where did your Grandad die? Hopefully, in Scotland???
Kind regards,
Tees