I, too, am researching the Garrett Hughes/Catherine Whelan tree. I suspect that Patrick Hughes, who is listed next to Garrett in the US Army enlistment document is a brother. A witness to the G Hughes/C Whelan marriage was a Patrick Hughes.
There is a Patrick Hughes who married a Mary McDermitt. They both died in Pennsylvania. They had one child that I know of, Mary.
I also saw that there is an Anthony Hughes residing next to Garrett in the 1860 census. Since Anthony is listed as 40 years old then, it may be that Garrett is an uncle. Could it be that the Anthony and your Catherine Hughes were siblings?
Thanks for any help you can share.