Hopefully answering your questions.
1. I think you have found the right marriage.
2. I can’t say for certain why Jane gave an age that wasn’t accurate. However it may have been because she didn’t want to admit to being older than her husband. Not something that would bother most couples today but was important in the early 1890s.
3. The address looks like 40 Sussex St.
4. The church was the Church of Ireland Mariners church. It was demolished some years back. It was in the docks area of Belfast (as the name might suggest).
5. Mark signed with his full signature. Jane signed with a cross, so she may have been illiterate. But that’s not certain as there’s quite a lot of discussion on this subject. People who signed with a cross on one document signed with their full name on another. No easy answer as to why they did that.
6. I suspect you got this certificate from GRO Roscommon, in which case it’s a clerk’s transcript. If you want to view the original, you need to go to the GRONI site.
https://geni.nidirect.gov.uk You will need to open an account and buy some credits. It costs £2.50 (sterling) to a view a certificate. It may show you how the couple actually signed.
7. Witnesses were Samuel McMillan and Mary Anne Wilson.
