Gods and Fighting Men, by Lady Gregory, , at sacred-texts.com
AND indeed Finn had no great luck in going to look for a wife that time; and he had no better luck another time he asked a wife from among the Sidhe. And this is the way that happened.
It was on the mountain of Bearnas Mor he was hunting, and a great wild pig turned on the hounds of the Fianna and killed the most of them, but Bran made an attack on it then and got the best of it. And the pig began to scream, and with that a very tall man came out of the hill and he asked Finn to let the pig go free. And when he agreed to that, the man brought them into the hill of the Sidhe at Glandeirgdeis; and when they came to the door of the house he struck the pig with his Druid rod, and on the moment it changed into a beautiful young woman, and the name he called her by was Scathach, the Shadowy One.
And he made a great feast for the Fianna, and Finn asked the young girl in marriage, and the tall man, her father, said he would give her to him on that very night.
But when night came on, Scathach asked the loan of a harp, and it was brought to her. One string it had of iron, and one of bronze, and one of silver. And when the iron string would be played, it would set all the hosts of the world crying and ever crying; and when the bright bronze string would be played, it would set them all laughing from the one day to the same hour on the morrow; and when the silver string would be played, all the men of the whole world would fall into a long sleep.
And it is the sleepy silver string the Shadowy One played upon, till Finn and Bran and all his people were in their heavy sleep.
And when they awoke at the rising sun on the morrow, it is outside on the mountain of Bearnas they were, where they first saw the wild pig.