The Cornish Language Partnership has a list of placenames in Cornwall in the Cornish language.
I have downloaded a shapefile with locations of places from www.mapcruzin.com which ultimately derives from OpenStreetMap. To this I have added the Cornish placenames, and plotted on a map in QGIS.
I still need to add in other features like rivers etc., and find something to make sure that estuaries like the Fal are shown as water rather than appearing to be dry land.
The colour scheme might look familiar if you remember the old Bartholomews 1:100k maps.