The story goes that the first inhabitants of Brugh na Bóinne ("Newgrange") were the goddess Bóinn and her 'husband' Nuadhu Nechtan, otherwise known as Elcmar, 'the envious one'.
The Dagdha, father of the gods, lusted after the goddess Bóinn. So he sent Elcmar on a journey which would take a night and a day, that he and the goddess Bóinn could spend the night together. The Dagdha then put a spell on the sun, causing it to stand still, stretching one day and night to nine months during which time the Dagdha lay with the goddess Bóinn, and she bore him a son, whom she called Aonghus 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 Dagdha had taken Aonghus to the Rath of Midhir where he was trained to be a champion hurler.
However, some years later, during a quarrel with another player, Aonghus Mac Óg learnt of his true parentage. At that time the Dagdha, was distributing the Sídhe among the gods, and Aonghus decided to go to the Dagdha to seek his rightful heritage. The Dagdha told him that there was nowhere left for him, but that he should go to Brugh na Bóinne at Samhain and demand possession of the Brugh from Elcmar for a day and a night.
This he did, but when the time was up, Aonghus refused to leave the Brugh, claiming that he had been promised 'day and night', of which eternity is composed.
Elcmar then called on the Dagdha to give his judgement over the situation; and the Dagdha ruled that 'it is indeed in day and night that all time is spent'. So, through a play on words Aonghus Mac Óg gained possession of the Brugh 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 prevailed upon him to act, his mother the goddess Bóinn was sent for, and she persuaded the Dagdha, his father, to send to all the lesser deities of Ireland, charging them to search for her. 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 thrice 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, and every year as soon as summer was over, she went with her companions to a lake called Lough Dragan, 'The Mouth of Sloes', and all of them became swans.
On the advice of the Dagdha, Aonghus went to the shore of the lake and waited in patience until "Samhain", the day of the magical change, and called to her. Caer appeared along with thrice 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 Brugh 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 the human form and they have lived happily there ever since.
