Bruce,
Here’s John Harris’s probate abstract from the PRONI website:
Harris, John of 76 Rugby Avenue Belfast retired salesman died 4 May 1963 at 34 Shanboley Road Santry county Dublin Administration Belfast 13 June to Mary Emily Harris the widow. Effects £2742 18s.
The probate abstract is simply a summary of his file. In this case it tells you he died intestate (administration) and so did not leave a will. His estate will then have been disposed of in accordance with intestacy laws. There should be a file in PRONI which should contain details of his surviving spouse & children. It’s not on-line, so if you want to know what it contains you either have to go in person, or pay a researcher. You can also ask PRONI to copy it but they are pretty expensive. I’d contact a researcher.
You can also view his death certificate on the GRONI website for £2.50.
https://geni.nidirect.gov.uk/
Here’s John & MInnie’s marriage certificate. They married in the West St Presbyterian church in Ballymena. Both worked at Holywell. That’s a mental hospital just outside the town of Antrim. It’s still open today.
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1919/09679/5510510.pdf
https://www.northerntrust.hscni.net/hospitals/holywell-hospital/
I see the following children to the marriage:
Eva born & died 20.8.1919 registered Antrim.
Martha 4.7.1921 Belfast
Elizabeth 30.5.1923 Belfast
In 1921 John was working in Belfast as a bread server (ie delivering fresh bread):
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1921/01178/1491131.pdf
There may have been other children but births within the past 100 years are not immediately accessible on-line. (You can search for them in PRONI or GRONI in Belfast but that involves a personal visit.).
Belfast Newsletter of 7th May 1963 contains a funeral message stating John (aka Jack) “died suddenly at his son-in-law’s home in Dublin. Deeply loved husband of Minnie Harris 76 Rugby Avenue, Belfast. Funeral from Brown’s funeral parlour, 292 Newtownards Rd today Tuesday 2.30 to Knockbreda cemetery. Deeply regretted by his sorrowing wife and family circle.”
If you were to contact Knockbreda cemetery they may be able to tell you if anyone else is buried with John Harris.
https://www.belfastcity.gov.uk/births-deaths-and-ceremonies/cemeteries/knockbreda-cemetery
GRONI has a marriage on 20.9.1947 in Belfast for a Martha Harris to John Greenway. You would need to view the certificate on the GRONI site (£2.50) to get the full details and see whether it’s your family. It's not a common name (the next previous Martha Harris marriage in Belfast was in 1937) so you might want to take a punt and see if this is John & Minnie's daughter.