Ian
I visited Cairnryan cemetery. It's very small, and there aren't many stones left to see. As you say it's right next to Loch Ryan. It's very well looked after, and there are a couple of war memorials there.
I still think the best bet for my Great-Grandmothers grave is Sheucan Cemetery, because of where she was living shortly before she died. There is no stone for her there either, but there are many stones missing.