The megalithic site at Brúgh na Bóinne, known in English as Newgrange , is strongly associated in mythology with the god Aonghus, a son of the father of the gods of the Tuatha Dé Danaan known as the Dagda. Aonghus was called "Mac Óg", or Young Son. This Irish equivalent of Eros was an eternally youthful exponent of love and beauty. Like his father, he had a harp, but it was made of gold, and not oak, as the Dagda's was; and so sweet was its music that no one could hear and not follow it. His kisses became doves which hovered invisibly over the young men and maidens of Ireland, whispering thoughts of love in their ears. The legend of Aonghus's birth, and of his eventual resting place, are both entwined with the ancient tomb of Newgrange.

The story goes that the first inhabitants of Brúgh na Bóinne were the goddess Bóann and her husband Nuada Nechtan, otherwise known as Elcmar, "the envious one". The Dagda, father of the gods, lusted after Bóann. He sent Elcmar on a journey which would take a night and a day, so that he and Bóann could spend the night together. The Dagda then put a spell on the sun, causing it to stand still in the sky, stretching one day and night to a duration of nine months, during which time the Dagda lay with Bóann, and she bore him a son, Aonghus, whom she called Mac Óg because "young is the son who is conceived at the start of the day, and born between that and evening". When Elcmar returned, believing that only a day and night had passed, he remained ignorant of events, as the Dagda had taken Aonghus to the rath (fort) of Midhir, where he was trained to be a champion hurler. However, some years later, during a quarrel with another player, Aonghus learnt of his true parentage. At that time the Dagda was distributing the Sídhe among the gods, and Aonghus decided to go to the Dagda to seek his rightful heritage. The Dagda told him that there was nowhere left for him, but that he should go to Brúgh na Bóinne at Samhain and demand possession of the Brúgh from Elcmar for a day and a night. This he did, but when the time was up, Aonghus refused to leave the Brúgh, claiming that he had been promised the "day and night" of which eternity is composed. Elcmar then called on the Dagda to give his judgement over the situation, and the Dagda ruled that "it is indeed in day and night that all time is spent". So, through a play on words, Aonghus gained possession of the Brúgh for all time.

Aonghus held splendid court at Newgrange . But one night he was visited in a dream by a beautiful maiden,who vanished when he put out his arms to embrace her. All the next day Aonghus took no food. Upon the following night, the fair apparition came again, and played and sang to him. That following day he also fasted. So things went on like that for a year, while Aonghus pined and wasted for love. Eventually the physicians of the Tuatha Dé Danaan prevailed upon him to act. His mother Bóann was sent for, and she persuaded the Dagda, his father, to send to all the lesser deities of Ireland, charging them to search for the girl. After a year she was found by Aonghus's brother, Bodbh the Red, who brought him to see her. Her name was Caer Ibormeith, meaning Yew Berry, and when Aonghus saw her, she was standing by a lake surrounded by one hundred and fifty maidens linked together by a silver chain. But when Aonghus asked her father for her hand in marriage he revealed that there was nothing he could do, as his daughter was a swan-maiden. Every year, as soon as summer was over, she went with her companions to a lake called Lough Dragan, "The Mouth of Sloes", and there all of them became swans. On the advice of the Dagda, Aonghus went to the shore of the lake and waited with patience until Samhain, the day of the magical change, and called to her. Caer appeared along with a hundred and fifty swans, herself a swan surpassing all the rest in beauty and whiteness, and promised to be his bride, if he too would become a swan. He agreed, and with a word she changed him into a swan. Together they flew three times around the lake, and took off side by side for Brúgh na Bóinne, where they put the dwellers of that place to sleep for three days and three nights with the magic of their singing. At Aonghus's palace they retook human form, and have lived happily there ever since.