Eithne is the mother of Lugh Lamhfada, the king of the Tuatha Dé Danann. She is the daughter of Balor, and as a prophecy proclaimed that Balor would be slain by his grandson, Eithne was locked in a tower of glass on Tory Island, that she would never meet a man.
Myths being what they are, this doesn’t go according to plan. Balor’s greed entices him to go after a magical cow that is always in milk, and the owner of this cow, Cian, seeks to get it back. Cian finds Eithne instead, and she becomes pregnant with the child Lugh. Variants of the tale have Cian also sleeping with Eithne’s 13 or 900 (!) handmaidens, who give birth to the race of seals.
When Lugh is born, Balor throws him into the sea to die, and he is taken and fostered by Manannán mac Lir, and later, Tailtiu. It is not said what happens to Eithne, though I believe that she is not freed until Lugh is grown and defeats Balor himself.
Eithne’s lonely imprisonment gives her years of time to wait, and watch, and oversee. She is the Pole Star around which everything turns, and yet, we forget she is there.
I’m uncertain how much Eithne and Arianrhod share; Arianrhod is more defined in Welsh myth, and receives more attention from neopagans in general than Eithne. Eithne is often a footnote, or considered simply Arianrhod’s equivalent, rather than with her own story and place. Even in relation to Lugh, she seems only a vessel, having spent little time with him before he is taken from her. It leaves Eithne independent; her captivity leaves her free to build her own skills of spinning and magic, overseeing and counting time, observing and contemplating.
She is an unseen pillar, from the still and silent Solstice when she births the promise child, and there she remains, holding space, until Lugh slays her captor. Then, when the door is flung open and she is free to leave… she would calmly sit back down by the fire, and resume her work.