Six Celtic nations based on their use of living Celtic languages: Scotland, Ireland, Isle of Man, Wales, Cornwall (England), and Brittany (France).
Celtic: The Celtic Nations. Only issue I have is it contains Cornwall, not Dumnonnia as a whole.
Celtic Nations--Ireland, Scotland, Wales, Cornwall, Spanish Galicia, Brittany, Isle of Man (Manx)
Celtic: Eight #Celtic Nations. This is a long read, plan on staying still for awhile. :) enjoy.
mud hut | Would you rather live in a mud-hut or an igloo?
Sgwd Dduli Waterfall Brecon Beacons National Park
Celtic Tribes of Roman Britain. Read non-fiction "How the Irish Saved Civilization" by Thomas Cahill and series of Irish history fiction books by Morgan Lewellyn
Llyn Idwal, Snowdonia, Wales by John Ormerod http://www.garreggochpark.co.uk/
I just found this! Beautiful wall map of Celtic nations--I WANT IT!! <3 (And the place names are shown in the native languages of their respective areas, plus the Latin names of ancient tribes!!!)
The ethnic groups in early medieval Britain. Image source: link