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I searched on the subscription site Roots Ireland and did not locate a baptismal record for young William or a marriage record for William and Catherine. The surnames would point to Northern Ireland (as your note indicated). I think at this point, following DNA matches is your best chance for coming up with more info on your family.
You may also want to add William and Catherine's story to our XO Chronicles site and possibly someone will see a connection. https://irelandxo.com/ireland-xo/history-and-genealogy/ancestor-database