According to Celtic myth, Angus (or Oenghus) is the son of Dagda and Boann. Conceived at daybreak, he was born between before the following day’s dawning. For this reason he is said to have power over time. He is the Mac ind Og, the Young God, who carried off Etain, the wife of Midir his foster father. In the story of Diarmaid and Grainne, he repeatedly saved his foster-son Diarmaid and his love from the vengeance of Fionn. After the Tuatha were defeated by the Milesians and the Sidhe were allotted, he was overlooked. He demanded to spend a day and a night in the Sidhe of his father Dagda at Brugh on the Boyne, but when the following day arrived and he was asked to leave, he replied “It is clear that night and day are the whole world, and it is that which has been given me.” Variations of this story claim he was given Bruig na Boinne in place of his mother’s spouse, Elcmar.
Historically, there was a king Oenghus of south Ireland during the fifth century. His name may be a mistranslation of Gaelic into Old English however. Anguisel is another variation of this king’s name.