You may know--and if you do, I'm sorry to be repetitive--that there are Irish naming patterns. Using this pattern might help you determine the names of the parents of Catherine and John. The first son was named after the paternal grandfather, the second after the maternal grandfather, the third after the father, the fourth after the father's eldest brother. The first daughter was named after the paternal grandmother, second after maternal grandmother, third after the mother, fourth after mother's eldest sister (sometimes this was reversed for daughters, with first daughter name after maternal granny, etc.).
The witnesses at a wedding were usually siblings of the couple. So were the witnesses at baptisms, at least until the couple ran out of siblings.
It looks like Catherine Grady had about seven siblings. You might want to look at their names and their witnesses to help further your research.