The son of Owain Ddantgwyn, a 5th-century ruler of the Kingdom of Gwynedd, and the brother of Saint Einion Frenin, a 5th or 6th-century king in the Llŷn Peninsula, Seiriol founded and governed a class (ecclesiastical settlement) at Penmon on the Anglesey.

Llywelyn the Great issued two charters in 1221 and 1237 confirming the Canons Regular, in possession of the island and the church and manor of Penmon on the mainland of Anglesey.

There is one identified shipwreck, a steamship named The Pioneer, which ran ashore in 1878 with a cargo of iron bars when the tow lines to it broke following its rescue from engine failure near the Skerries.

A programme of poisoning these rats started in 1998 by the Countryside Council for Wales appears to have eradicated them, and the puffin population has shown an increase since that date.

[11] At one time the island was heavily grazed by rabbits, but these were wiped out by an outbreak of myxomatosis, leading to the growth of dense vegetation, particularly common elder (Sambucus nigra).

The abundance of fish feeds a large colony of grey seals, and bottlenose dolphins and harbour porpoises regularly visit the area.