Thanks for the suggestions Christopher.
Unfortunately if the only clue I have about the families origins is via Scottish censuses and all they say is Sligo, so I don't really have much to go on. I know a lot of us with Irish roots are in the same boat.
Since posting my original question, I have discovered the names of Thomas Weir's parents: Thomas Weir (farmer) and Isabella Dow. Elizabeth/Bridget Green's parents are James Green (portioner) and Mary surname unknown.
I have thought about the fact that the surnames Weir, Green, and Dow don't appear to be typical of, or specific to Ireland, and wonder if the families origins are elsewhere?
Thanks again.