In Goidelic mythology, Ethlinn was the daughter of Balor, the god of death.

According to a prophecy, Balor was to be killed by his grandson. He locked Ethlinn 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.

Ethlinn 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 and also because the language developed and changed from Archaic Irish through Old Irish, Middle Irish and Modern Irish.

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 Mac Diancecht. Her son was Lugh Lámhfada. Her grandsons were Cúchulainn and Fionn Mac Cumhaill. Because of Ethlinn's connections the three different races of Formorians, 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 Ethlinn is still famous today, as the Queen of New Age singers Enya is named after her.