The 8th-century text known as
The Expulsion of the Déisi attributes to the Corcu Duibne an eponymous founder, Corc Duibne, a scion of the branch of the Érainn royal line called the
Síl Conairi, after Conaire Mór. In particular, the later "B version" of the text includes a lengthy episode describing Corc's birth and childhood deeds. Corc and his twin brother Cormac are born of incest to Coirpre Músc (a quo Múscraige) and Duihind, children of
Conaire Cóem, a descendant of Conaire Mór. Their conception causes the crops to fail, and the people determine to
immolate them to remove their curse. However, a
druid steps in and offers to take Corc to an offshore island so that the abomination is out of Ireland. Reciting a poem predicting great things for Corc's descendants, the druid and his wife Boí take the boy to the remote island of Inis Boí. Every morning for the next year, Boí performs a purification ritual in which she gives Corc an
ablution while he is seated on the back of an
otherworldly white cow with red ears. Finally one morning Corc's curse leaves him and enters the cow, who jumps into the ocean and turns to stone, becoming the rock of Bó Boí. Boí takes Corc to his grandmother, Sárait, daughter of
Conn Cétchathach, and eventually convinces her to take him back. When he is older Corc is sent to serve as hostage in the court of
Cormac mac Airt,
King of Tara. There he is fostered by Óengus Gaíbúaibthech, a leader of the
Déisi. When Óengus and his people are expelled from Tara over a bloody dispute with the king's son, Corc absconds from hostageship and joins his foster-father, fighting beside him in many battles. Eventually the Déisi wander to the southern coast, and come to the island where Corc was reared. He tries to convince them to settle there, but they elect to move farther north. Corc remains, and founds his dynasty. ==Annalistic references==