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

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, one of the Tuatha de Danaan, with the help of the druidess Birog, managed to enter the tower. She gave birth to a son, Lugh, by him, but Balor threw him into the ocean. Birog saved him and gave him to Manannan mac Lir, who became his foster father.

Ethniu is a fine example of the difficulty of conducting research into Irish mythology. Her name has been variously spelled as Ethlinn, Ethnea, Eithliu, Ethlend, Ethnen, Ethlenn, Ethniu, Ethnenn, Ethne, Aithne, Enya, Eithne, Aine, Ena, Etney, Eithnenn, Eithlenn, Eithna, Ethni, Edlend, Edlenn. This is due to the lack of a standardised spelling in ancient Ireland, because the language developed and changed from Archaic Irish through Old Irish, Middle Irish and Modern Irish, and because of various attempts to anglicise the name.

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.

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 Cruaich, 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.