Thanks so much for your help with this, I now have Mary Jane Mitchell/Darroch's death certificate with her parents names (Peter Darroch & Anna McKenna). To get any further i'll need to get to grips with Irish records, which i've never had to use before. We had no idea there was any Irish on this side of the family so it's really interesting!
Similarly with James Mitchell - his parents are James Mitchell and Mary Murray, both Irish. And even Alexander Brown.
On another note, do you know have any sort of idea how frequent it is that a couple wouldn't have a marriage record in Scotland or why that would be the case?