Des,
I would recommend you also check out Shane's suggestion. It's pretty unusual to have a marriage cert which doesn't give father's names and fuller addresses for either party. I think you've probably got a copy of the cert the priest gave them, whereas the full civil register entry should give you more family information. You can get a photocopy for €4 from GRO Roscommon, so it's worth checking out. (Irish records can be thin on the ground, as you've probably discovered, and so you need to pursue every slender lead).
Elwyn