Boann's second great story is a domestic one, though no less strange for it. Married to Nechtan, called Elcmar in some tellings, she was loved by the Dagda, chief of the Tuatha De Danann, and the two arranged an affair while her husband was away. To hide the child that would come of it, the Dagda sent Elcmar on an errand and made the sun stand still for nine months, so that Boann conceived, carried, and gave birth to their son within what Elcmar experienced as a single day, returning to find no time had apparently passed at all.
The son born of that concealed day was Aengus Og, later a god of love and poetic inspiration in his own right, and the deception did not end with his birth. Aengus grew up fostered elsewhere, and on reaching adulthood tricked his own father out of his home at the great mound of Newgrange on the Boyne by asking to stay there for a day and a night, then arguing that since all time is made of days and nights, he was owed the dwelling forever. The Dagda, who had once bent time itself to hide his son's conception, lost his home to the same kind of wordplay.
The story ties Boann permanently to the Boyne valley's most visible monument, since Newgrange stands directly on the river she is said to have made, and Irish tradition treats mother, river, and passage tomb as three expressions of the same sacred landscape rather than three separate facts placed near one another by chance.