In Irish mythology, Ethniu (Eithne, Ethliu, Ethlinn, and a variety of other spellings - see below) was the daughter of Balor, king of the Fomorians.

She is best known as the mother of Lug. Her union with Lug'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. According to a prophecy, Balor was to be killed by his grandson. He locked Ethniu in a tower made of crystal to keep her from becoming pregnant. However, Cian, with the help of the druidess Birog, managed to enter the tower and seduce her. She gave birth to triplets, but Balor threw them into the ocean. Two either drowned or turned into seals, but Birog saved one, Lug, and gave him to Manannan mac Lir, who became his foster father.

She is a central figure in the mythology because so many of the gods and heroes are related to her. Her father was Balor. Her father-in-law was the Dagda. Her husband was Cian, son of Dian Cecht. Her sons were Lugh Lámhfada and Delbáeth. Her grandsons were Cúchulainn and Fionn mac Cumhail. Because of Ethniu's connections the three different races of Fomorians, Tuatha Dé Danann and Milesians became united in her descendants.

Ethniu is a fine example of the difficulty of conducting research into Irish mythology. Her oldest version of her name is probably Ethliu or Ethniu, giving rise to the modern Irish name Eithne. However thanks to changes in the Irish language, the lack of standardised spelling for many centuries, and attempts to anglicise the name, variations have arisen. Linguistic ignorance has further confused the issue: the genitive form, as in mac Ethlenn ("Ethliu's son"), has often been taken for a nominative, or a mistaken nominative has been inferred. Such variations include: Ethlinn, Ethnea, Eithliu, Ethlend, Ethnen, Ethlenn, Ethnenn, Ethne, Aithne, Enya, Aine, Ena, Etney, Eithnenn, Eithlenn, Eithna, Ethni, Edlend, Edlenn.

Although most of the texts state she was the daughter of Balor, there are two different pedigrees in the ancient text named 'Baile in Scáil'. Therein one version states Lugh Lámhfada is "Lug mac Ethlend maic Tigernmais" and another states "Ethnen m.Smretha m.Tigernmais". James Bonwick, in his book "Irish Druids and Old Irish Religions" (1894), equates Tigernmas with Balor. This seems to be borne out by the Baile in Scáil reading. As Tigernmas is credited with introducing the worship of the god Crom Cruach, it seems to connect Balor and Crom in some way.

The name Ethniu is still famous today, as the Queen of New Age singers Enya is named after her.