Etain in Irish mythology or Éadaoin is a Mythical lady of the Tuatha de danann.
One source mentions a poetess of the Tuatha Dé with this name while another introduces her as an otherworld helper of the Munster king Eoghan Mór in his travails. Her name was based on ét (later, éad), meaning ‘envy’, and thus designated ‘she who evokes envy’. Her original significance can therefore be seen as that of the Goddess of sovereignty who leads to rivalry between aspirants to Kingship.
She is featured in a very colourful story, written in the ninth century. According to this, after Aengus had got possession of Brú na Bóinne (the Newgrange tumulus in County Meath), his foster-father Midir went to visit him there.
Midir attempted to separate two fighting youths, and had one of his eyed knocked out by a holly prick in the struggle. Diancecht came and healed him, but he nevertheless demanded compensation from Aengus
What he sought was the most beautiful maiden in Ireland. Éadaoin Eachraidhe, daughter of Ailill, king of Ulster. Aengus went to Ailill, who demanded that twelve plains be cleared for him and twelve rivers be created. All of this was done by Aengus‘ father, the Dagda, and after Aengus gave Éadaoin’s weight in gold and silver, he was allowed to bring her home with him.
…see also Eithne.
Extract from: Ó hÓgáin, D. (2006). The Lore of Ireland: An Encyclopaedia of Myth, Legend and Romance. The Boydell Press. pp. 189-190
Etain in Irish Mythology:
Tochmarc Etain – The Wooing of Etain
Altrom Tige Dá Medar. – The Fosterage of the House of the Two Pails.
Some secondary sources:
Carey, J. 2016, ‘The final transformation of Étaín’, Ériu, 66, pp. 31-38.
With Pedersen, Camilla Michelle. 2015. Metamorphoses: A Comparative Study of Representations of Shape-Shifting in Old Norse and Medieval Irish Narrative Literature. Master’s thesis, National University of Ireland Maynooth. pp. 61.
Monaghan, P. 2008. The Encyclopedia of Celtic Mythology and Folklore. New York: Checkmark Books. pp. 162