Elves and the Tuatha de Danaans

[Bill Ferny]

Lalaith is correct. Welsh, Breton and the extinct Cornish were not spoken in Ireland. Did Tolkien know Gaelic?

However, this is splitting hairs. Both Goidhelic and Brythonic languages have a common Celtic root. At any rate, one does not have to know the language to know the mythology, and the mythologies of the Goidhelic speaking Celts is intimately related to the mythologies of the Brythonic speaking Celts. JRR Tolkien, if he had knowledge of Celtic mythology, would have had knowledge of Irish Celtic mythology whether he knows Goidhelic languages or not. Remember, the Leabhar Gabhala was first compiled and written in medieval Latin.