In the Mythological Cycle of early Irish literature, Midir (Old Irish) or Midhir (Modern Irish) was a son of the Dagda of the Tuatha Dé Danann. After the Tuatha Dé were defeated by the Milesians, he lived in the sidh of Brí Léith (believed to be Ardagh Mountain, Co. Longford). In the First Recension of the Lebor Gabála, Midir of Brí Léith is made the "son of Induí son of Échtach son of Etarlam".
Midir is one of the leading characters in the Old Irish saga Tochmarc Étaíne ("The Wooing of Étaín"), which makes leaps through time from the age of the Túatha Dé Danann to the time of Eochaid Airem, High King of Ireland. Midir was the husband of Fúamnach, but later fell in love with Étaín, receiving the help of his foster-son and half-brother Aengus (also Oengus) to make her his new bride. This provoked Fuamnach's vengeance against the young new wife, causing her a number of disgraces until after several transformations (including water, a worm, and a butterfly or dragonfly) Étaín fell into the drink of another woman and was reborn. She later married Eochaid Airem, at that time the High King of Ireland. Far from giving up, Midir made an attempt to bring his wife back home, going to see the king and challenging him to many games of fidchell. Eochaid won all but the last, when Midir won and asked a kiss from Étaín as his prize. Eochaid kept his word and allowed Midir the kiss, but Mider turned himself and Étaín into swans and left the royal residence through the chimney. Eochaid did not accept the loss of his wife and pursued them. Then Midir used his magical powers to turn fifty women into similar to Étaín, offering the king the possibility to choose only one. Eochaid, trying to find the true one, chose his own daughter by accident and lost Étaín, also fathering a daughter upon his own daughter in the process.
Midir figures in a brief anecdote about the stingy poet Athirne, son of Ferchertne, in the heroic age portrayed by the Ulster Cycle. The story, entitled Aigidecht Aitherni ("The Guesting of Athirne") in one manuscript, recounts that Athirne came to Midir's house in Brí Léith and fasted against him so that he obtained from him his three magical cranes which stood outside his house denying entry or hospitality to anyone who approached. Moreover, "[a]ny of the men of Ireland who saw them could not face equal combat on that day."
Midir also interfered when Fráech attempted to woo Treblainne.
- Tochmarc Étaíne ("The Wooing of Étaín"), tr. J. Gantz. Early Irish Myths and Sagas. London: Penguin Books, 1981. ISBN 0-14-044397-5.
- Aigidecht Aitherni ("The Guesting of Athirne"), ed. and tr. Rudolf Thurneysen, "Zu irischen Texten. I. Athirne von seiner Ungastlichkeit geheilt." ZCP 12 (1918): 389-9; ed. and tr. Kuno Meyer, "The Guesting of Athirne." Ériu 7 (1914): 1-9; ed. R. Thurneysen, "A third copy of the Guesting of Athirne." Ériu 7 (1914): 196-9 (diplomatic edition); tr. John Carey, "Athairne's Greediness." In Celtic Heroic Age, ed. J.T. Koch and J. Carey. 3d ed. Aberystwyth, 2000. MSS: (1) LL 117a, (2) MS Harleian 5280, fo. 77 (alt 66) and (3) Royal Irish Academy, 23 N 10, pp. 15–16.
- Tochmarc Treblainne, ed. Kuno Meyer, "Tochmarc Treblainne." ZCP 13 (1921): 166-75; tr. R. Jennings, "A translation of the Tochmarc Treblainne." Emania 16 (1997): 73-8.
- Uhlich, Jurgen. "Einige britannische Lehnnamen im Irischen: Brenainn (Brenden), Cathair/Cathaer und Midir." ZCP 49-50 (1997–98): 878-97.