In Irish mythology, Ethniu (Old Irish pronunciation: [ˈeθʲnʲu]) or Eithne (Irish pronunciation: [ˈehnʲə]); also Ethliu, Ethlinn, and a variety of other spellings – see below) is the daughter of the Fomorian leader Balor, and the mother of Lugh.

Her union with Lugh's father, Cian of the Tuatha Dé Danann, is presented in early texts as a simple dynastic marriage, but later folkore preserves a more involved tale, similar to the birth of Perseus in Greek mythology. A folktale recorded John O'Donovan in 1835 tells how Balor, in an attempt to avoid a druid's prophesy that he will be killed by his own grandson, imprisons Ethniu in a tower on Tory Island away from all contact with men. But a man called Mac Cinnfhaelaidh, whose magical cow Balor stole, gains access to Ethniu's tower, with the magical help of the leanan sídhe Biróg and seduces her. Ethniu gives birth to triplets, but Balor gathers them up in a sheet and sends a messenger to drown them in a whirlpool. The messenger drowns two of the babies, but unwittingly drops one in the harbour, where he is rescued by Biróg. She takes the child back to his father, who gives him to his brother, Gavida the smith, in fosterage. The boy grows up to kill Balor. By comparison with texts like Cath Maige Tuired and the Lebor Gabála Érenn, the unnamed boy is evidently Lugh, and his father, Mac Cinnfhaelaidh, is a stand-in for Cian.The Banshenchus states that her real name was Feada- "Feada was the real name of noble Ethne who was wife of strong stout Cain, and mother of Lug the impetuous superman, and daughter of swift smiting Balor son of Dod son of mighty Net a greater man than pleasant Hector. From him is famed the cairn at Ath Feindead because he fought a duel."