Tradition tells us that Iaun was a nobleman of royal descent from "beyond the seas", probably Dumnonia. Feeling the pressure of raids from Frisians settling in Wessex, he fitted out a fleet and crossed with his followers to Cornouaille. He is said to have established a reputation as a just ruler and his people gave him an epithet meaning Law-Giver. He was eventually succeeded on the Throne of Cornouaille by his son, Daniel Dremrudd.

Records of Iaun Reith date back to the 12th century. He could well be historic.