Puffin Island When St. Seiriol founded the Penmon monastery he also set up a community on a small island just off the coast, 1/2 mile from the priory. This island is called Puffin Island in English but its Welsh name is Ynys Seiriol (Seiriol's Island). The Vikings called it Priestholm.

The island has a number of ruins of medieval monastic buildings, including the tower of a 12th century church. It is said that St. Seiriol himself is buried there, and perhaps also King Maelgwn Gwynedd, who was the ruler of North Wales and patron of St. Seiriol in setting up the religious community.

The island is mentioned by Gerald of Wales in his Journey through Wales in 1188. He notes that it was an ecclesiastical settlement at the time, "inhabited by hermits, living by manual labour and serving God". He also says that, according to legend, whenever there was strife within the community a plague of small mice would devour all their food.

A modern day plague of rats has also affected the island. As the name suggests, Ynys Seiriol once had large numbers of puffins as well as other seabirds such as guillemots. However, brown rats found their way to the island in 1890s, decimating the populations of nesting birds. At this time the puffin population was already reduced because the birds had become a delicacy, but the rats further reduced their numbers to just 20 pairs a few years ago. In 1998 a program was begun by the Countryside Council for Wales to rid the island of the rats, in the hope of encouraging the birds to return.

The sound between Puffin Island and Penmon Point (or Trwyn Du) is treacherous. A ship called the Rothesay Castle, on a day trip from Liverpool, sank here in 1831. A lighthouse and a lifeboat station were built here soon after.