I thought that eos would be a bit like the Irish gaelic dheas which could be translated as sweet ,but thanks for that correction guest . I tried looking up the words on a couple of Cornish on-line translation sites - I tried Breton ones as well on the off chance , but there were no entries for either eos or whek .

The song had been sang at one time in Cornish and the English song we all know and love was a translation which was then re-translated into Cornish ,presumably by language revivalists , sometime in the 20th century . My only source for all this is the Kennedy book which quotes one of the lead miners who Marje mentioned , Cap'n John Stocker ,saying that he had heard the song sang by his grandfather who had died at the age of 100 . The English version was first written down in 1857 by Robert Bell who had heard it from these Cornish miners in Germany . The song itself has very definite links to Cornwall and was very well known there ,but wasn't known to any great extent (as far as I am aware) outside of Cornwall until more recently .

A few questions for any Cornish speakers on mudcat:
Did the language revivalists invent the word Eos or did they use ,as guest suggests , a word that meant something like small bird ? Is there a generic word in Cornish for "smallbird" . If not ,what sort of small bird did they use as a substitute for nightingale ? Does eos translate as thrush ,blackbird or some bird that did exist in Cornwall ?
It all seems a bit convoluted to me and the simplest solution would be that , whatever about Berekley Square , the sweet nightingale was reasonably well known in Cornwall at some time , and that it was called an eos whek . Perhaps it died with the language .