Saint Sithney was a sub-Roman Celtic hegumen who lived during the sixth century. Originally from Ireland, he was part of a group of monks who traveled to western Cornwall and Brittany. He is associated with several monastic traditions, including time spent at Inis Mor and Saighir. Following his service in Ireland, he relocated to Brittany, where he established a significant presence. He died in 529 in Guissény, and his legacy remains tied to both the Cornish and Breton regions.