I don't think FamilySearch would have been allowed access to the GRO registry books, if they have an image it's likely to be just of the index book and not the cert.
Sort of, but also sort of not. The LDS Family History centers have had microfilms of the actual marriage registrations since at least the early 1990s. They have the marriage certs for all of Ireland from 1864 (maybe 1845), to 1870. I know, as I spent many an hour ordering and peering at these microfilm records.
However, not all the FHC microfilms have made their way online onto FamilySearch, so you may be strictly correct.
