[Saint] Dyfnan is reputed to have been a son of Brychan Brycheiniog, but his name is not found in either version of the Cognatio. He is the patron of Llanddyfnan, in Anglesey, where he is buried, according to tradition.
You would imagine, due to the proximity of the church to the stone, that there would be a story to connect the stone with the saint. But I don't know of one.. Surely there's one out there somewhere.
p396 of Sabine Baring-Gould's 'Lives of the British Saints', part 3. 1907