Etain was taken as a second wife by Midir, an Irish god of the Underworld.* This made Midir's first wife Fuamnach rather upset, and so she turned Etain into a butterfly. Midir then throws Fuamnach out. Etain then flys into the mouth of a queen; nine months later, the queen gives birth to the human Etain. She falls in love with King Eochad Airem and marries him. They have a daughter named Etain, who bears a striking resemblence to her mother.
However, all this time, Midir has searched for Etain. Finally he found her at Eochad's castle. Etain now realizes who she really is, and wants to return to the Hollow Hills. However, Eochad is devistated, and refuses to let her go. Midir challenges him to a game of chess (in Irish, the game is fidchell, which is similar to chess). Eochad looses, and Midir leaves. However, Eochad sends his troops out after them and captures them. Back at the castle, Midir transforms all the attending women into the likeness of Etain--even the daughter Etain--and tells Eochad to pick the true Etain. If he does so, Midir will never return to bother him. Eochad makes his choice, takes her to the bedroom, and sleeps with her while Midir leaves--with the real Etain. Unfortunately for Eochad, he picked his own daughter, on whom he now committed incest. Midir and Etain return home to be together forever.
However, the result of this incestuous union is the doomed king Conair Mor, from the story The Destruction of Da Derga's Hostel.
*When I say "underworld" I am being literal, as opposed to the land of the dead. That is, the Tuatha de Danaans
lived in the hollow hills, which was also the abode of the blessed dead (as is Tir na nOg
, though it doesn't have the same connotation as Tartarus
, as the dead are usually reincarnated in Celtic myth).