From “Fionn Mac Cumhail and the Son of the King of Alba”
Myths and Folk Tales of Ireland, by Jeremiah Curtin, 1890, p 203 ff.
http://www.gutenberg.org/files/36540/36540-h/36540-h.htm

Then the cowherd went away to Cahirciveen in Kerry where he had an enchanted castle. But before he went he invited Fionn MacCumbail and the Fenians of Erin to have a great feast with him. "For," said he to Fionn, " I'm not a cowherd at all, but the son of the king of Alba, and I'll give you good cheer."
When the Fenians came to the place, they found the finest castle that could be seen. There were three fires in each room and seven spits at every fire. When they had gone and sat down in their places, there was but one fire in each room.
"Rise up, every man of you," said Fionn, " or we are lost; for this is an enchanted place."
They tried to rise, but each man was fastened to his seat, and the seat to the floor; and not one of them could stir. Then the last fire went out and they were in darkness.
"Chew your thumb," said Conán to Fionn, "and try is there any way out of here." Fionn chewed his thumb and knew what trouble they were in. Then he put his two hands into his mouth and blew the old-time whistle. And this whistle was heard by Pogán and Ceolán, two sons of Fionn who were in the North at that time, one fishing and the other hurling.
When they heard the whistle, they said: " Our father and the Fenians of Erin are in trouble." And they faced towards the sound and never stopped till they knocked at the door of the enchanted castle of the son of Alba at Cahirciveen.
"Who is there? " asked Fionn.
"Your two sons," said one of them.
"Well," said Fionn, " we are in danger of death to-night. That cowherd I had in my service was no cowherd at all, but the son of the king of Alba; and his father has said that he will not eat three meals off one table without having my head. There is an army now on the road to kill us to-night. There is no way in or out of this castle but by one ford, and to that ford the army of the king of Alba is coming."
The two sons of Fionn went out at nightfall and stood in the ford before the army. The son of the king of Alba knew them well, and calling each by name, said: "Won't you let us pass?" "We will not," said they; and then the fight began. The two sons of Fionn MacCumhail, Pogán and Ceolán, destroyed the whole army and killed every man except the son of the king of Alba.
After the battle the two went back to their father. "We have destroyed the whole army at the ford," said they.
"There is a greater danger ahead," said Fionn. "There is an old hag coming with a little pot. She will dip her finger in the pot, touch the lips of the dead men, and bring the whole army to life. But first of all there will be music at the ford, and if you hear the music, you'll fall asleep. Now go, but if you do not overpower the old hag, we are lost."
"We'll do the best we can," said the two sons of Fionn.
They were not long at the ford when one said, "I am falling asleep from that music." "So am I," said the other. " Knock your foot down on mine," said the first. The other kicked his foot and struck him, but no use. Then each took his spear and drove it through the foot of the other, but both fell asleep in spite of the spears.
The old hag went on touching the lips of the dead men, who stood up alive; and she was crossing the ford at the head of the army when she stumbled over the two sleeping brothers and spilt what was in the pot over their bodies.
They sprang up fresh and well, and picking up two stones of a ton weight each that were there in the ford, they made for the champions of Alban and never stopped till they killed the last man of them; and then they killed the old hag herself.
Pogán and Ceolán then knocked at the door of the castle.
"Who's there?" asked Fionn.
"Your two sons," said they; " and we 'have killed all the champions of Alban and the old hag as well.''
"You have more to do yet," said Fionn. "There are three kings in the north of Erin who have three silver goblets. These kings are holding a feast in a fort to-day. You must go and cut the heads off the three, put their blood in the goblets and bring them here. When you come, rub the blood on the keyhole of the door and it will open before you. When you come in, rub the seats and we shall all be free."
The three goblets of blood were brought to Cahirciveen, the door of the castle flew open, and light came into every room. The brothers rubbed blood on the chairs of all the Fenians of Erin and freed them all, except Conán Maol, who had no chair, but sat on the floor with his back to the wall. When they came to him the last drop of blood was gone.
All the Fenians of Erin were hurrying past, anxious to escape, and paid no heed to Conán, who had never a good word in his mouth for any man. Then Conán turned to Diarmuid, and said:
"If a woman were here in place of me, you wouldn't leave her to die this way." Then Diarmuid turned, took him by one hand, and Goll MacMorna by the other, and pulling with all their might, tore him from the wall and the floor. But if they did, he left all the skin of his back from his head to his heels on the floor and the wall behind him. But when they were going home through the hills of Tralee, they found a sheep on the way, killed it, and clapped the skin on Conán. The sheepskin grew to his body; and he was so well and strong that they sheared him every year, and got wool enough from his back to make flannel and frieze for the Fenians of Erin ever after.