Dyen,
GRONI is the only repository for birth, death and marriage records in what is now Northern Ireland, from 1.1.1922 onwards. (Ancestry and other websites don’t have them).
Births in Northern Ireland within the past 100 years, marriages within the past 75 and deaths within the past 50 aren’t searchable on-line. They are open to the public, just not on-line. If you know all the key details eg exact date, couples names etc, as you evidently do in the case of Cormac and Morna’s marriage, you can order a copy of a certificate from GRONI on-line for £15 (sterling).
https://geni.nidirect.gov.uk
Alternatively, if you live near Belfast, you can go in person to either GRONI or PRONI (both in Belfast). They are the only places where you can search recent records, right up to those registered yesterday. You can view each cert there for £2.50, and if you want a certified copy it’s an extra £8. (Posted out in 2 to 3 weeks time).