Thanks for updating. Have been wondering how things were going.
If only we knew how old Gabrielle was at the 1933 wedding or in the 1939/40 register. I know that there is a birth record for a 'Mary Gabriel Donegan' in 1924, but I have also found a very strange one for a Mary G. Donegan in Dublin North in 1912. The trouble is, that is all it says - there is no quarter shown, nor a reference number.
Are you going to send for the 1924 certificate? Even if just to rule it out?