In addition to the tale of a tunnel connecting this site and Caerau:
It is said to have been the abode of one IOAN ; but whether he was saint or sinner is not known. On one occasion it is said that, when pursued by the enemy, he crossed the stream, and left the impression of the hoof of his charger on the stone, which has something like the mark of a horse's shoe upon it.
The old chronicler of Caerau, who used to say that he had been baptized by a vicar of St. Dogmaels (dead since 1768), and who had spent almost all his lifetime on the farm of Penrallt Ceibwr, was alive a few months ago. He told me that the whole neighbourhood was considered "fou." That men were led astray there all night, not knowing whither they went until cock-crowing, when they discovered that they were not far from home (hence the white gate-posts). A man carrying a bundle of hoop-rods, in one of these midnight wanderings, dropped them one by one to ascertain the extent of his journey ; and when he went after them in the morning, he found he had travelled an incredible number of miles. A St. Dogmaels fisherman having been to a wedding at Moilgrove, lost his bearings on his way home at night, and was for some hours not able to find his course, until at last he fortunately discovered the north pole (? the polar star), by which he sailed homewards ....
This, however, cannot be said of them all; for an old clerical friend of mine of sober habits, had once the honour of joining in this magic dance for the great part of a night. All the land round Caerau was once unclosed, which may account, in some measure, for these vagaries.
From 'The History of St Dogmael's Abbey' by Emily Pritchard (1907), who was actually quoting the Rev. Henry Vincent in Archaeologia Cambrensis, Oct. 1864. The book also mentions some local stoney folklore, about the Sagranus ogham stone, which served for ages as a bridge over the aforementioned stream and with a White Lady crossing over it at midnight, and no-one wanted to touch it after dark. Then it was mysteriously transferred to the vicar's wall, and afterwards to the church.