While re-reading the story of Taliesin, I ran once again into the mystery of the whereabouts of the sunken kingdom of Gwyddno Garanhir. There seemed to be a couple of good clues as to its location. First, a chieftain named Seithenhin is mentioned in connection with the place. Second, he is said to be buried between a Caer Genedr and the sea.
Seithenhin is said to be from Latin Septentinus and this may well be correct. However, I would see it as a substitution for 'the Setantian', i.e. an eponym of sorts for the Setantii tribe. This people, according to Rivet and Smith (see THE PLACE-NAMES OF ROMAN BRITAIN), lived between the Mersey (ancient Seteia) and Fleetwood at the mouth of the Wyre.
Genedr looks suspiciously like the River Kent in Cumbria, from an earlier Kenet, probably plus dwr, 'water'. The 'Caer' in question is probably the Roman fort of Alauna on the Kent just a little south of Kendal. The River Kent, like the Wyre of the Setantii, empties into Morecambe Bay, which is described thusly:
"It is the largest expanse of intertidal mudflats and sand in the United Kingdom, covering a total area of 310 km2 (120 sq mi). [https://en.wikipedia.org/wiki/Morecambe_Bay]
By saying that Seithenhin's grave was between the fort on the Kennet and the sea, we are being told that it is in Maes Gwyddno/Morecambe Bay, the Sunken Land.
That Gwyddno may have been relocated to Wales in later legend is suggested by his apparent identification with a Man of the North. This entry is from P. C. Bartram's A CLASSICAL WELSH DICTIONARY: