Wow. Many thanks.
I never seen this on FS for Ireland before, but found it through the new search engine. That'll teach me to use the old one. Alexander's father was Peter so it matches
So there is a good chance that Isabella was of Irish stock, which would give me two irish ancesters on my mom's side. I'd looked for the marriage in military marriage records for India, Africa etc and all sorts of places.
So I have another certificate to send for. I remember reading that some Dublin records were lost during the uprising after WW1, so I hope it is still available.