I looked at the GRONI on-line marriage indexes (which at present go up to October 1942). I searched for a John Boyd marriage to an Ellen, in the Lisburn registration area but didn’t see one. (Culcavy and Hillsborough are both in the Lisburn registration area). When I widened the search to all of Northern Ireland there were too many John Boyd marriages to make it possible to identify the right one. And of course he could have married anywhere, outside Northern Ireland too. Tradition was to marry in the bride’s church, so though John may have lived near Culcavy, we don’t know where Ellen came from.
I’d be inclined to get the will from the 1946 probate file and see if any children are named. Then with their names, it should be possible to trace their births and get the mothers maiden name and marriage details that way.
Children’s births in Northern Ireland within the past 100 years are not on-line. The restriction is to prevent data mining. The records are open to the public but you will either need to get someone to go to GRONI or PRONI in person (they are the only 2 places where you can look more recent births up), or you can get in touch with GRONI by e-mail or phone and get them to search for you. There might be a search fee depending on what’s involved.