Hi Maureen,
I realised that I had a GRO record for a 1904 ProCathedral marriage so have just compared them.
The date is the date of marriage in both these cases (but the date when the registration was confirmed on the GRO record is almost a month later so gives the possibility that a different date could be copied across).
In the record that I checked, the addresses of both bride and groom have been changed slightly in the church record book, the number is different on one and a "Gt" has been missed off the other address, and the bride's family name has an O' prefix missing in the church record.
I suspect that the church entries were entered at some time after the event and there appears to have been a lack of care taken in doing so (perhaps on the basis that this was not a legal record). What I was asking earlier was: is the GRO copy a certified copy or a research copy? The research copy, if it is the same as the one I obtained from Dublin research rooms, would have an extra column at the beginning showing when the formal register entry recorded at the church was registered by the registrar. It's possible that it was the date from that column that was accidentally entered in the church record book (which has many entries on every page - as opposed to the registrar's book at the church which would have only two entries per page, I think).
Sheila