[galgot]

Could it be that the Ys city legend in Brittany was this Welsh legend transposed in Brittany, when the Bretons (originally Welsh peoples) migrated there... very interesting. https://en.wikipedia.org/wiki/Ys

[Nursie]

And don't forget Lyonesse, a Cornish lost land, with related language and people - https://en.wikipedia.org/wiki/Lyonesse