Daughter of Lir, who, with her brothers, was turned into a swan by her jealous stepmother Aoife, in the story The Fate of the Children of Lir, one of The Three Sorrows of Storytelling.
For nine hundred years, she and her brothers wander the lakes of Ireland, singing to all who would listen, until they were baptised by one Saint Mac Howg.
It is possible that Fionnuala and her brothers are ultimately derived from the Indo-European concept of the third-function twin brothers, and their sister the sun/swan maiden. Other examples are the Dioskouri (Castor and Pollux) and their sister Helen, or Oengus and Caer.
Thomas Moore, the eighteenth-century Irish poet, wrote "The Song of Fionnuala" based on this myth in his Irish Melodies.