Hi Looby,
I was feeling guilty about forgetting to look out for your ggg grandparents, so I used a couple of spare credits I had for the Scotlands People website.
Thomas Dorman is listed as having died in Leswalt Parish, in 1864 - age not given.
Found two Annie Dormans who died in Wigtown County around that time, one age zero, died in 1899 at Penninghame, Wigtown, and the other aged 20, died at Stranraer (other surname given as Stevenson). The next nearest didn't die until 1913 (Annie, other surname Little, aged 54, in Stranraer), and 1916 (Annie Elizabeth, aged 47, in Stranraer).
Could your Annie have moved from the area after Thomas died?