Francis,
The 1883 Port Glasgow marriage certificate tells you that John McCaughey was dead by that date. Mary was still alive. He was presumably alive when his son Hugh was born in 1863, so I searched for deaths from 1864 – 1883 in Co. Derry. I found one in 1873 that might fit except that the occupation is labourer (whereas the marriage certificate says he was a farmer). The couple lived in Moyagall, near Gulladuff. Parish of Maghera.
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1873/020701/7263313.pdf
Birth registration started in Ireland in 1864. I searched for children born 1864 onwards to this couple but did not find any more. The Catholic parish records are on Ancestry and I searched them for the couples marriage and the baptisms of their children but did not find the family at all. However not every parish has records pre 1864 and so that may be the explanation.
There were no McCaugheys in Moyagall in the 1831 census, nor Griffiths Valuation c 1860 nor in the 1901 census. That’s not unusual, particularly for labourers who moved about to follow the available work. It just makes them difficult to trace. I searched for a death for Mary Dunlop but again without success.
I looked at the Valuation Revision records for Moyagall 1864 – 1881 to see if the McCaughey family were listed. They were not. That could mean they were lodging with someone or that they didn’t live there very long and never made it into the records. (Griffiths clerks revisited every couple of years so if you didn’t stay that long you would not be in the records).
I can’t say the Moyagall couple is the right one, just I can’t see any others.
Elwyn