No 'Cornwall' at Coedcernyw, Gwynllwg (modern Monmouthshire)

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:

https://historicplacenames.rcahmw.gov.uk/placenames/headname/44200347-a60f-4f92-bf31-d43405564a35

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:

http://www.coflein.gov.uk/en/site/307811/details/gwern-y-cleppa-long-barrow

We can dispense with the notion, therefore, that this supposed Welsh "Cornwall" had anything whatsoever to do with Arthur.