Fergnae mac Óengusso Ibdaig (died 557) was a Dal Fiatach king of Ulaid.
[2] He was slain at the Battle of Druim Cleithe (Kilclief, County Down) by his cousin Demmán mac Cairill (died 572) who had support of the Uí Echach nÁrda, another branch of the Dal Fiatach.
[3] As a result of this battle, his branch of the Dal Fiatach known as the Ui Ibdaig were excluded from the throne.
Some propose that it was named for Fergnae's father's mother,[5] while others suggest that Ibdaig is an epithet—the Hebridean—applied to his father, who should be identified with the eponymous ancestor of the Cenél nÓengusa of Islay.
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