Iestyn's dates of birth and death are not recorded. According to some sources, he flourished in the early 6th century; according to others, he was active in the 7th century. He is said to have been the son of Geraint ab Erbin, a ruler of
Dumnonia (a Celtic kingdom in what is now southwest England). His brothers were recorded as including
Cador, Duke of Cornwall, and
Cyngar (another saint who is commemorated in the name of the church at
Llangefni, Anglesey). Iestyn, a hermit and confessor who was probably a follower of the Anglesey saint
Cybi, founded two churches in north Wales: one,
St Iestyn's Church, Llaniestyn, Gwynedd, and another
St Iestyn's Church, Llaniestyn in
Anglesey. He may also have founded the church at
St Just in Roseland, in a part of modern-day
Cornwall within his father's influence, and has also been linked with a church in
Brittany, in northern France. His
feast day is not included in the Welsh calendars of saints, but was marked at his church on Anglesey on 12 April and 10 October, and at his church in Caernarfonshire on 10 October. ==See also==