The Children of Lir (Oidheadh Chlainne Lir)
Nine hundred years as swans, and the bell that ended it.
Lir had four children, Fionnuala, Aodh, and the twins Fiachra and Conn, and when their mother died he married her sister Aoife. Aoife came to hate the children for the love their father gave them, took them to a lake, and struck them with a druid's wand into the shape of swans.
She left them their human voices and their reason, which is the cruelty of it. They would keep both for nine hundred years: three hundred on Loch Dairbhreach, three hundred on the Sea of Moyle between Ireland and Scotland, and three hundred at Iorras Domhnann in the west. The spell would break only when they heard the bell of a new god.
The first three hundred were bearable, because their father and the Tuatha Dé could come to the lakeshore and talk with them. The Moyle was not. It is open, cold water, and they were separated by storms and spent the centuries looking for each other, with Fionnuala sheltering her brothers under her wings.
When the nine hundred years were up they came ashore and heard a bell, because Christianity had arrived in Ireland while they were on the water. A monk took them in. The spell broke, and the four of them became what they actually were: people more than nine centuries old. They were baptised and they died at once, and were buried together.
The tale is one of the Three Sorrows of Storytelling, with the sons of Uisneach and the children of Tuireann.