You can search online for records of your grandparents in the 1901 and 1911 census returns. You can check them out at /www.census.nationalarchives.ie/search/. This assumes that your John and Bridget were living in Donegal on these dates. This should allow you to identify the location in which they lived at that time.
It's quite possible that there is still family at these locations, as farms frequently stay in the ownership of the same family from generation to generation.
From a quick search, I can see at least 2 families with John and Bridget Canny as parents in the 1901 returns - maybe one of these couples are the grandparents you seek.