My family was there last summer (2003). The castle in Scotland is located near Leswalt. If you have a good map, like a Thomas guide, it will actually show you the location of it. Nearby is also the Tor of Craigoch, or Monument Hill, a conspicuous tower, 60 feet high and 20 square at the base, erected in 1850 to the memory of Sir Andrew Agnew (1793-1849).
Anyway, at the time that we were there, the local townspeople in Stranraer all advised us against going up to the castle property, telling us that it was now privately owned. After having lunch in Agnew Park, in Stranraer, a very nice park by the way, we decided to take a drive up to Leswalt anyway and check out the situation ourselves. We found that the property, which was vacant at the time, was in the process of being restored, and there was scaffolding all around it. We were able to respectfully walk around the castle and take pictures.
I'm not sure if the house is completed now, or if it is occupied. Good luck!