Newgrange and Mound B in twilight mist

Síd in Broga (Newgrange) on the top of the ridge at Brú na Bóinne, with Dagda's Mound (Mound B) beside the river Boyne shrouded in fog in the foreground. In mythology (De Gabail In t-Sida/The Taking of the Mound), Oengus Óg (the Mac Óc) asks Dagda to allow him to stay for a night and a day inside Newgrange. Dagda allows it, but when he returns next day to regain possession of the great mound, Oengus tells him "the world passes in nights and days" and that, in fact, permanent possession of the Brug has been given to him. Dagda apparently acquiesces to this trickery, and goes instead to a smaller (but still substantial) mound nearer to the Boyne, labelled Mound B on modern archaeological maps. After this, Síd in Broga ceases to be known by this name and is instead known as Síd Mac Ind Óc, the mound of the Young Son, because, according to the story Tochmarc Étaín (about Oengus), "Young is the son who was begotten at the break of day and born betwixt it and evening".