The Children of Lir were cursed to live as swans for 900 years.
In the Irish legend, Aoife transforms Lir’s four children—Fionnghuala, Aodh, and the twins Fiacra and Conn—into swans through jealousy of their father’s affection. The spell divides their punishment into three periods of 300 years.
They spend the first 300 years on Loch Dairbhreach, followed by 300 years on the Straits of Moyle between Ireland and Scotland. Their final 300 years are spent at Iorrus Domnann and Inis Gluairé. They retain human speech and become renowned for their beautiful, sorrowful singing.
The total is therefore 300 + 300 + 300 years, not 1,000 or 500. At the end of the curse, Christianity has reached Ireland; the aged swans are restored to human form only briefly before they die and are baptised and buried.