Liam, I too, have O'Callaghan, in my family, be it distantly, by marriage.
I wonder if your O'Callaghan, may have dropped the O, in USA, as some of mine did. If this is possiblity, maybe you should post same message dropping the O?
I also wonder if you have any associated surnames with O'Callaghan/Sexton? That may help some.
I'd like to try to find a connection
Mine came from Cork, and Donegal areas of Ireland.
If I can help I'd be glad too, just ask specific questions so I'd know what I was looking for.ok?

Thank you
Barbara