The Dagda had an affair with Boann, wife of Nechtan. In order to hide their affair, the Dagda made the sun stand still for nine months; therefore, Aengus was conceived, gestated and born in one day. Midir was his foster-father.
When he came of age Aengus dispossessed the Dagda of his home, the Brú na Bóinne (an area of the Boyne River Valley that contains the Passage tombs Newgrange, Knowth and Dowth). He arrived after the Dagda had shared out his land among his children, and there was nothing left for Aengus, so Aengus asked his father if he could live in the Brú for a day and a night, and the Dagda agreed. But Irish has no indefinite article, so "a day and a night" is the same as "day and night", which covers all time, and so Aengus took possession of the Brú permanently. In a different version of this story, appearing in "The Wooing of Étaín", Aengus uses the same ploy to trick Elcmar out of Brú na Bóinne, with the Dagda's connivance. In this version, Midir is Aengus's foster-father, while Elcmar is the husband of Boann cuckolded by the Dagda.
According to the Death Tales of the Tuatha de Danaan, Aengus killed his step father Elcmar for killing Midir.
Aengus also slew the poet of Lugh Lamfada for lying about his brother Ogma an Cermait. The poet claimed that Ogma was having an affair with one of Lugh's wives. Aenghus killed the poet in front of Midir.
In the Wooing of Etain, Aengus was able to partially lift Fuamnach's spell against Etain, the horse goddess he had won for his brother Midir.Fuamnach in a jealous rage had turned the girl into a butterfly. Turning her into a woman at night, Aengus made her his lover until Fuamnach found out about it and drove her away. Aengus killed his foster mother for her treachery.
In the Tale of the Two Pails, a sidhe woman and foster daughter of Aengus gets lost and winds up in the company of St. Patrick. The girl converts to Christianity, and Aengus can not win her back. He leaves, and she dies of grief a few weeks later.
In the Battle of Ventry Aengus helped defend the battle weary men of Finn mac Cumhail against Roman invaders.
There are also tales of him being a powerful shapeshifter in the stories of Cuchulainn, his great-grandson, and Finn mac Cumhail, his nephew.
Aengus fell in love with a girl he had seen in his dreams. His mother, Boann, searched Ireland for an entire year. Then his father, the Dagda, did the same. Finally, King Bodb Dearg of Munster found her after a year.
Aengus went to the lake of the Dragon's Mouth and found 150 girls chained up in pairs. He found his girl, Caer Ibormeith. On November 1, Caer and the other girls would turn into swans for one year, every second Samhain. Aengus was told he could marry Caer if he could identify her as a swan. Aengus succeeded. He turned himself into a swan and they flew away, singing beautiful music that put all its listeners asleep for three days and nights.
He owned a sword named Moralltach, the Great Fury, given to him my Manannan mac Lir. This sword he gave to his foster-son Diarmuid.There was also a sword named the Little Fury and two spears of great power that he gave to Diarmuid. When the young man died, Aengus took his body back to the Brugh where he breathed life into it whenever he wanted to have a chat.
There are other legends that he was able to repair broken bodies and return life to them.Old Irish name Óengus is attested in Adomnan's Life of St. Columba as Oinogus(s)ius, showing that its etymology is from the Proto-Celtic roots *oino- "one" and *guss- "choice".