Turissa: I just came across a slip of paper with some notes from when I did research in County Down three years ago. I noticed the name "Nancy Shilliday" in a database (the same name as my great-great grandmother). I realized she was not the Nancy I was seeking, but now realize she is yours. All the information you posted is consistent with what I had jotted down, except that the marriage date I have is Jan. 12, 1852.
Also, the marriage record indicated that Nancy Shilliday was from the townland of Drumlee. That may help in tracing Hugh Shilliday's family -- you can check this against the Griffith's Valuation Survey, Tithe Applotment Survey and other land records.