I've seen several different spellings for the name even within the family so I wouldn't worry too much about that. From what I can tell though, Patrick's sister was born in Roscommon, Ireland which is some distance from Belfast. The family moved to Northumberland, England sometime in the late 40s but then spent a few years in New York in the early 60s and appear on the electoral records there. I am not sure if Mary went to America with them as I can't access records at the minute. When I next subscribe to ancestry, I'll look into it a bit more for you.
I hope this helps,