There is a considerable amount of information about Saint Piran available on the World Wide Web, with some degree of consensus, but much is unproven legend - not very surprising since it appears there are no contemporaneous records.
   The story seems to start in the sixth century when he was a priest in Ireland.   He was noted for his ability to perform miracles, a talent which caused some heathens in the area to become very frightened of him, so much so that they eventually tied him to a millstone and rolled it over the cliff into a stormy sea. The result was not as they expected.  The sea immediately became calm, and the millstone floated.  The currents took him away and he was eventually washed up on the Cornish coast at Perranporth.
   There he founded his first Christian Church on the mainland, and allegedly re-established the mining of tin in the region, an industry which had existed in the past but had been forgotten long before his arrival. This came about when he noticed that as his black hearthstone became hot, the white molten tin in the rock came to the surface, forming a cross.  He passed on his discovery of this phenomenon to the locals and the industry was revived.  He later became the Patron Saint of Tin Miners.  The pattern made by the tin on the hearthstone is said to be the origin of the Cornish flag.
   His status as patron Saint of Cornwall is generally accepted, but there are other claimants to the title, including  St. Michael and St. Petroc.  A day of celebration for St. Piran is held annually on March 5th.