Mongán mac Fiachnai

Mongán mac Fiachnai (died ca. 625) was an Irish prince of the Cruthin, a son of Fiachnae mac Báetáin. Little is certainly known of Mongán's life as only his death is recorded in the Irish annals. He appears as a character in the Cycles of the Kings where he is said to have been the son of Manannán mac Lir and perhaps a reincarnation of the legendary hero Finn mac Cumaill of the Fenian Cycle.

The tale Compert Mongáin (the Conception of Mongán), which survives in three variants, has Mongán fathered on Fiachnae's wife Cáintigern by the sea-god Manannán mac Lir while Fiachnae campaigned alongside Áedán mac Gabráin of Dál Riata. The versions have different accounts of how this came about, all of which agree that some form of bargain was struck whereby Fiachnae's life was saved by Manannán in return for a night with Cáintigern. An early version of this tale is found in the Immram Brain where Manannán prophecies Mongán's birth and near divine nature to Bran. Although the surviving versions of the tale are from the 10th or 11th century, earlier versions are believed to have been included in the lost Cín Dromma Snechtai manuscript. The verses in which the claims of Mongán's divine parentage and tutoring are made are described by Charles-Edwards are "literary conceit" and by Carney as "poetic hyperbole", the presumed original intent being to vaunt Mongán's seamanship.

Manannán takes Mongán away with him to Tír Tairngire—the land of promise, an otherworld similar to Tír na nÓg—where he learned shapeshifting and other esoteric knowledge. While Mongán is in the otherworld, his father is killed by Fiachnae mac Demmáin, an event which the Irish annals place after Mongán's death. Mongán's ability to change his shape is alluded to in the 9th century tale De Chophur in dá Muccado (The quarrel of the two swineherds), found in the Book of Leinster, which is one of the stories setting the scene for the Táin Bó Cuailnge.

...
Wikipedia