Adam
Pure speculation from my part now

...that needs to be firmed up as mentioned by Ev and Rosie with David's marriage reg to Hannah and confirmation of his father.
I am wondering whether David was already down in England by the time of the 1841 census. There is this entry showing in Great Boughton, Wirral:
Thomas Marshell 40 agr. lab. b. Ireland
Mary Marshell 12 b. Ireland
David Marshell 7 b. Scotland
Thomas Marshell 6 b. Cheshire, England
I thought these entries might fit in England for possibly Thomas and wife Agnes:
Thomas above?
https://familysearch.org/pal:/MM9.1.1/JM1J-6F4Agnes maybe died before census?
https://familysearch.org/pal:/MM9.1.1/F3JL-7BCThere also shows these bann/marriage entry for a couple with these names in Wigtownshire:
https://familysearch.org/pal:/MM9.1.1/XY7L-6WRhttps://familysearch.org/pal:/MM9.1.1/XY7L-7PVDifferent parishes normally indicate bride and groom's residence at the time of the marriage (they weren't always born there but banns were read at last residence).
As I mentioned, all guesses for now...

Monica