This is a short story from P.W. Joyce's 'The Wonders of Ireland' (1911):
On one occasion, as the poet Erard Mac Cossi was standing on the shore of the river Boyne, he saw a flock of wild swans flying past very near him. Taking up a round stone, he flung it into the midst of them and struck one on the wing, so that it fell to the ground helpless and fluttering, while the others flew off. The poet ran immediately to catch it, but when he came up he found it was not a swan but a woman dressed in white. As soon as he had recovered from his astonishment he spoke to her and asked how it happened that she had been flying about in the shape of a swan. She replied that some short time before, she had a sudden heavy fit of illness, and that she grew rapidly worse, till one day, when she was lying at the point of death, a number of demons came into the house and carried her off silently, while it appeared to her friends that she had died. She and the demons took the shape of swans, and from that out she remained flying about with them from place to place, till the poet set her free by a lucky accident.
Mac Cossi then brought her to his house and treated her kindly, and after a little while restored her to her friends.
The above story is yet another example of a swan story connected with the Boyne River, which in Irish is Bó Finne, the white cow, and may have been seen as an earthly Milky Way in ancient times. See the Cygnus Enigma for more information on the swan connection.