In the book The Spirit Paths of Wales by Laurence Main - Cicerone Press 2000 the os grid reference for St Mary's Well is given as SH.139253. Main goes on to say "There is still a path to a curious holy well in a gulley below the church". "This is covered by the sea at high tide, yet contains fresh water from a spring at low tide". The well also called Ffynnon Mair was the last port of call for pilgrims before their journey across Bardsey Sound to the holy island itself. The author says that just above the well and ruined chapel is an ancient stone called Maen Melyn (Yellow Stone). The stone is covered with yellow moss, if that's where the name comes from.
Something is not right. This message is just to keep things from messing up down the road