The "Cornwall'" or Cernyw/Kernyw thought by many to be present at Coedcernyw in Monmouthshire is not, as it turns out, a cernyw name at all.
It is instead merely Coed + carnou, the plural for the word carn, "cairn, barrow, tumulus, mound, rock; heap, pile."
We have evidence for this in early forms for the place-name:
Plus, the geography fits Coed-carnou, while there is no 'horn' or anything else which would merit the cernyw name.
Not only are Great Pen-carn and Middle Pen Carn and Pen Carn nearby in the parish, but also in the parish is a great stone chambered tomb:
We can dispense with the notion, therefore, that this supposed Welsh "Cornwall" had anything whatsoever to do with Arthur.