I wish Leila were online to confirm or refute this but I think her ancestor Cornelius McCarthy was born 1818, son of Andrew McCarthy and Ellen Hegarty. I have not heard this from Leila and my main reason for this conclusion is because the first son of Cornelius was named Andrew. Cornelius and his wife Mary Collins also had a daughter Margaret who had a godfather Bat Hegarty.
My working model for now is that this Cornelius, son of Andrew, was a first cousin of my ancestor Cornelius. These two Corneliuses had families that amazingly resemble each other in records. Both had wives named Mary. Both had daughters named Mary, Ellen, Margaret. In fact, when I first started researching my Lissane McCarthys, I almost had Skibbereen Heritage retrieve the wrong family but realized my mistake because of the age difference in the two Ellens. The main reason why I was able to more confidently distinguish the two families is because my relatives told me that my great-grandmother Margaret "had many sisters" but there was never any mention of brothers. Your Cornelius McCarthy had sons.
You can view my McCarthy tree at my own family history page, which includes the branch you are researching. http://www.sjbar.com/irishGenealogy/
If you have information that would correct/clarify these McCarthys, I would appreciate knowing. I am still researching my own branch and trying to distinguish it from another branch in the area. Other than that there are just too many McCarthys in that general area to be able to confidently state that A was a brother of B, so I don't really have any further info other than what is in my tree, which remains rather speculative.