Part I Book IV: Aine

Gods and Fighting Men, by Lady Gregory, [1904], at sacred-texts.com

AND as to Aine, that some said was a daughter of Manannan, but some said was the Morrigu herself, there was a stone belonging to her that was called Cathair Aine. And if any one would sit on that stone he would be in danger of losing his wits, and any one that would sit on it three times would lose them for ever. And people whose wits were astray would make their way to it, and mad dogs would come from all parts of the country, and would flock around it, and then they would go into the sea to Aine's place there. But those that did cures by herbs said she had power over the whole body; and she used to give gifts of poetry and of music, and she often gave her love to men, and they called her the Leanan Sidhe, the Sweetheart of the Sidhe.

And it was no safe thing to offend Aine, for she was very revengeful. Oilioll Oluim, a king of Ireland, killed her brother one time, and it is what she did, she made a great yew-tree by enchantment beside the river Maigh in Luimnech, and she put a little man in it, playing sweet music on a harp. And Oilioli's son was passing the river with his step-brother, and they saw the tree and heard the sweet music from it. And first they quarrelled as to which of them would have the little harper, and then they quarrelled about the tree, and they asked a judgment from Ollioll, and he gave it for his own son. And it was the bad feeling about that judgment that led to the battle of Magh Mucruimhe, and Oilioll and his seven sons were killed there, and so Aine got her revenge.