I am right now researching the same Ellen McCarthy. I believe she was my Granddad's maternal aunt. I was told by my first cousin once removed Michael Collins of Bunalun that his dad's mom (and my granddad's mom) "had many sisters" and that "one of them was named Ellen."
My Granddad's mom was Margaret McCarthy. Her father was Cornelius McCarthy of Lissane. Her mother was very likely Mary Collins. Her husband was Humphrey "Humphrey Mickey" Collins. Granddad was one of their kids. She died in 1906 after giving birth to their 10th child.
I am aware of Margaret, Mary, and Ellen among those siblings. I have just asked Skibbereen Heritage Center to search the baptismals for *all* those McCarthy children.
You can see my family documents here:http://www.sjbar.com/irishGenealogy