Lir is an old irish word for the sea but in this story it is the name of a widowed king who remarries and his sorceress wife is jealous of his love for his four children.  She changes them into swans and curses them to live on all the oceans of the world for 900 years.  The intervention of monks returns them to human form but they die and for me it is a tale of a new religion replacing an old one and all the loss involved.