Thanks for taking the trouble to look, Elwyn. It's a disappointing result, but it seemed worth a try.
I see that you posted a similar message in your exchange with Peter Mackey on irelandxo.com. There was a Samuel Mackey, born 6 September 1756, who was a brother of Thomas Mackey, born 16 October 1746, the father of Martha McLorinan, wife of Henry McLorinan, my 3xg-grandparents. The age is right for it to be his grave that you found, and maybe other Mackeys are in unmarked and no longer marked graves. That Mackey family also figures on a tree on Ancestry.com called Original Robinsons in Dominica; I haven't tried to fathom the Robinson connection, but the death in Dominica that you found on one of the graves you saw fits.