There are numerous Cornelius & Mary Hickey's in my tree. My gg grandfather Cornelius married Elizabeth Holland, her sister Edmund married Catherine Hickey and their sister Johanna married a different Cornelius Hickey. Johanna & Cornelius Hickey have a son Eugene, perhaps there is another Eugene in the family.
The notes I have on Nellie are handwritten by a relative who is not longer with us, so no one to verify the data... Her notes say that Catherine had 4 sisters and 1 brother, John, Dolly, Mary, Nellie & Betty (I presumed that Betty was Elizabeth).
The notes in the book show that John married a Joan, that is all that is written, so perhaps that is wrong... Could the Elizabeth living with Catherine & Edmund Holland be her sister? I have that she married a Patrick Haley, but nothing more. This is not my direct lineage so I don't have much. Another note on Nellie that may help, her daughter Mary married a Phillip Edward Barry 1821 - 11 Sep 1899, their children are Ellen, John, Mary, Philip, Mark, Bernard & James. I do have some more details on the children if you want them.