Maybe a marriage cert for Grace and Amos would be a good place to start, as that's your confirmed connection. A cert should show father's names and occupations, although sometimes occupations are not mentioned for a deceased father, the cert should at least still include his name. Places of residence and names of witnesses are also included on civil records, which can also help with connections..
the likely references for the marriage are as follows :
Name: Grace C V French
Quarter & Year : Jan-Mar 1920
Registration district: Bandon
Volume: 5 / Page : 3
Bandon town and the registration district is located between Cork city and Skibbereen.
One of the possible matches is Amos John Blackall. To order research certs for any of the items mentioned, you can download the order from from the HSE website here Print this out and fill in your details, and then make sure to include the references, as above for the marriage, and then post or fax to the GRO. (Dont worry about leaving some sections of the form blank, it's designed to be used for more recent certs for passports etc)
Edit : make sure to select 'photocopy' option on the order form (section 3) for the cheaper research certs - these cost just 4 Euro.