She is the daughter of Ailill, king of the Ulaid. "Of her was said: 'Shapely are all till compared with Etain,' 'Dear are all till compared with Etain.' When Midir of the Tuatha Dé Danann falls in love with and marries her, his rejected first wife Fuamnach becomes jealous and casts a series of spells on her. First Fuamnach turns Étaín into a pool of water, then into a worm, and then into a beautiful butterfly. Midir does not know that the butterfly is Étaín, but it becomes his constant companion, and he has no interest in women. Fuamnach then creates a wind that blows the butterfly away and does not allow it to alight anywhere but the rocks of the sea for seven years.
Eventually it lands on the clothes of Aengus, who recognises it as Étaín, but he is at war with Midir and cannot return her to him. He makes her a little chamber with windows so she can come and go, and carries the chamber with him wherever he goes. But Fuamnach hears of this and creates another wind which blows her away from him for another seven years. Eventually the butterfly falls into a glass of wine. The wine is swallowed (together with the butterfly) by the wife of Étar, an Ulster chieftain, in the time of Conchobar mac Nessa. She becomes pregnant, and Étain is reborn, one thousand and twelve years after her first birth.
When she grows up, Étaín marries the High King, Eochaid Airem. Their meeting related in the opening episode of The Destruction of Da Derga's Hostel. Eochaid's brother Ailill Angubae falls in love with her, and begins to waste away. Eventually he admitts to Étaín that he is dying of love for her, and she agrees to sleep with him to save his life. They arrange to meet, but Midir casts a spell which causes Ailill to fall asleep and miss the assignation. However, Étaín meets a man there who looks and speaks like Ailill but does not sleep with him because she senses that it is not actually him. This happens three times, and the man who looks like Ailill reveals himself to be Midir, and tells her of her previous life as his wife. She refuses to leave with him unless her husband gives her permission. She then returns to Ailill to find him cured.
Midir then goes to Eochaid in his true form and asks to play fidchell, a board game, with him. He offers a stake of fifty horses, loses, and gives Eochaid the horses as promised. Midir challenges him to more games, for higher stakes, and keeps losing. Eochaid, warned by his foster-father that Midir is a being of great power, sets him a series of tasks, including laying a causeway over Móin Lámrige, which he performs reluctantly. He then challenges Eochaid to one final game of fidchell, the stake to be named by the winner. This time, Midir wins, and demands an embrace and a kiss from Étaín. Eochaid agrees that he will have it if he returns in a month's time. A month later Midir returns. He puts his arms around Étaín, and they turn into swans and fly off.
Eochaid and his men begin digging at the mound of Brí Léith where Midir lives. Midir appears to them and tells Eochaid his wife will be restored to him the following day. The next day fifty women who all look like Étain appear, and an old hag tells Eochaid to choose which one is his wife. He chooses one, but Midir later reveals that Étaín had been pregnant when he had taken her, and the girl he has chosen is her daughter. Eochaid is horrified, because he has slept with his own daughter, who became pregnant with a girl. When the girl is born she is exposed, but she is found and brought up by a herdsman and his wife. She later becomes the mother of the High King Conaire Mor.
A slightly different genealogy is told in The Destruction of Da Derga's Hostel. Here she is the daughter of Étar, and marries the High King Eochaid Feidlech. They have a daughter, also called Étaín, who marries Cormac, king of Ulster. She bears him a daughter, Mess Buachalla, but no sons. Cormac abandons Mess Buachalla, but she is found and brought up by a herdsman. When she grows up she marries the High King Ederscel and becomes the mother of Conaire Mor.