Just offshore from Black Point lies Puffin Island, or Ynys Seiriol. This island has been known by a large number of names, including Priestholm, Ynys Lannog, and Ynys Glannauc. Priesthom reflects the island's monastic history, as does Ynys Seiriol, Seiriol being a sixth century saint.

There is a church dedicated to Saint Seiriol at Penmon, with an adjoining Priory. A nearby fishpond would have been a source of food for the monks, and past the fishpond is St Seiriol's Well, or Ffynnon Seiriol.