The tale supposedly took place long ago before even the Gaels arrived in Ireland, which predates the Norman invasion by over a thousand years and more.
The Da Dannan were considered to be early supernatural beings in Ancient Ireland and when Bodh Derg was elected as the high king, King Lir was very angry. Bodb married off one of his daughters, Aoibh, to Lir and they were very happy together .having four beautiful, soft skinned children called Fionnuala, Aodh and Fiachra and Conn, who were twins.
The family were heart broken when Aoibh died and so Bodb sent another daughter, Aoife, to marry Lir. Aoife cared for the four beautiful at first but soon became jealous of the love their father had for them especially as she had no children with Lir of her own.
One day, Aoife took the children out in her chariot with some servants and after a short distance, she ordered the servants to kill the children but the servants could not bring themselves to do that.
Aoife ordered the children to go down to the lough where she lifted her wand and turned them into beautiful snow-white swans. The curse was that they spend 300 years swimming on Lough Dervarragh, 300 years swimming on the Sea of Moyle (in the north of Ireland) and 300 years swimming in the Great Western Sea (Atlantic). The curse would only be broken and the children would only be free when a bell sounded.
When Lir discovered what had happened to his beloved children, he took Aoife to Bodh the High King who in his fury turned Aoife into a demon of the air.
900 years passed and the swans were swimming in the water when they heard the Christian bell brought by Patrick. Patrick wanted to bless them but the children knew that they would be 900 years old when the curse was lifted. They asked Patrick to baptise them first and so Patrick baptised the swans. The spell was lifted and the children regained human form but died immediately of old age.