There is a marriage of an Ann Flett to Hendrey Sutherland in 1819 in Firth and Stennes on Orkney. This is so far the only possibility I've found.
I suspect he worked at the Flagstone works in Castletown
Recorded as labourer, stoneworks in 1841 census
Recorded as labourer in 1851 census.
Also recorded in 1851 census living with them on Castletown Sibmister Road is a grand-daughter, Elisabeth McKay, age 1, born in Olrig.
Presumably a lodger, George Hercus, a mason age 40 is recorded at the same address in 1841 census
Death register entry records him as stone dresser and his father as mason (master)
1851 census gives age as 51 and place of birth as Orkney.
1841 census gives age as 40 and birthplace as outside of county.
This makes the most likely candidates for parents as John Flett and Molly Card - place of birth being Stromness in 1799 - which funnily enough is Mabel's suggested date!
Henry Sutherland Death Cert.
Mabel Bruce, Mabel's notes.
Mary Ann Cormack Birth Record.