Piran’s first oratory was probably built of wattle and daub which was not very durable so a new one of stone was built at a later date. It is the remains of this one that now lies buried in the sand.

Replica of St. Piran’s Oratory (Royal Institution of Cornwall)
Excerpts from “In the Shadow of Saint Piran” by Eileen Carter
People loved their saint so much that they wanted to be, in death as in life, as close to him as possible and so the oratory thrived for centuries. During this time sand threatened the building. There is nothing written down and we can only surmise what happened. Was there a big sand bar at sea? Or dunes further into and across the bay, the sea level being lower? Here again spare time for thought and speculation. There is certainly an inconceivable amount of sand all along the north Cornish coastal areas. The threat became so intolerable that the people were forced to consider another site for the new church.
Piran too well brooketh his name in Sabuloe: for the light sand carried up by the north wind from the sea shore daily continueth his covering and marring the land adjoinant, so as the distress of this deluge drave the inhabitants to remove their church, Howbeit when it meeteth with any crossing brook, the same (by a secret apathy) restraineth and barreth his farther encroaching that way. It was in consequence of this notion that the inhabitants, thinking such situation secure, removed their church only about 300 yards, it being on the opposite side of the brook.
(Richard Carew)
This must have been a very traumatic decision for them, to abandon the place of their beloved saint; but eventually nature decided for them, and a new site was decided upon, a few minutes walk to the east, across a small stream. Still within sight of the oratory, the people took heart, knowing that the towering dunes could not cross water.
We cannot be sure that the oratory building now reburied in sand is the original structure of St. Piran. Some academics believe that stone was not in use in the 6th century, but the very hallowed spot Piran chose would have been where the little oratory was built. Whatever the argument of time scale, no one can say with certainty, so once again we must ponder and make up our own minds: 5th, 6th, or 7th centuries: does it matter? We know that a date of great antiquity lies here. we also know that a graveyard of immense proportions is buried under the sand.