..I walked across to Criccieth Station; but on my way I was directed to call at a farm house called Llwyn y Mafon Uchaf, where I was to see Mr. Edward Llewelyn, a bachelor then seventy-six years of age. He is a native of the neighbourhood, and has always lived in it; moreover, he has now been for some time blind. He had heard a good many fairy tales.From chapter three of
.. He told me of a man at Ystum Cegid, a farm not far off, having married a fairy wife on condition that he was not to touch her with any kind of iron on pain of her leaving him for ever. Then came the usual accident in catching a horse in order to go to a fair at Carnarvon, and the immediate disappearance of the wife. At this point Mr. Llewelyn's sister interposed to the effect that the wife did once return and address her husband in the rhyme, Os bydd anwyd arfy mab, &c.
Celtic Folklore Welsh And Manx
by John Rhys
 (online at the sacred texts archive).
Surely the fairies had something to do with the cromlech - it can't be coincidence that the husband came from that farm?
The 'usual incident' is that the husband had tried to throw a bridle over his horse, but accidentally touched his fairy wife with it. The rhyme is some motherly advice for the children she'd left behind:
"If my son should feel it cold,
Let him wear his father's coat;
If the fair one feel the cold,
Let her wear my petticoat."
Posted by Rhiannon
26th March 2007ce