Who were the Tuatha de Danann?
Were they historical figures turned to legend? Fairies? Gods? Their truths slide and change depending on the story and the time the story was written. I recently came across the story of the Children of Lir which wasn't written until the late medieval period and seems to have been created for the purposes of a morality tale.
We start with a rivalry between two of the Tuatha de Danann, Lir and Bodb Derg, son of the Dagda. To keep the peace, Bodb offers Lir one of his foster daughters, Aobh, as a wife for LIr. Things go well between she and Lir. She presents him with two sets of twins: a girl, Fionnghuala, and a boy. Aodh, and then two additional boys, Fiachra and Conn. Unfortunately, she dies in childbirth with the second set of twins so Bodb offers another of his foster daughters as a replacement wife, Aoife. But, as the classic evil stepmother, Aoife is jealous of the love shown to the children. She demands that her servant kill the children. The servant refuses and Aoife lacks the courage to kill them herself so using her powers of sorcery she transforms the children into swans.
When Bodb learns what his daughter has done, he turns her into a demon of the air as punishment. Meanwhile, the unfortunate children spend the next 900 years as swans until, at long last, they meet a monk named Mochaomhog. He blesses them and as they accept the blessing and convert to Christianity, the spell is broken and they transform back into humans. Alas, though, their advanced age instantly catches up with them and they die in that moment, but their souls ascend to heaven.