The story can be found here: http://tentacle.net/~dawnhawk/witchcraft/cmyth2.html

"Cumhaill mac Bascna was chief of Clan Bascna. A tall, sturdy, blond-haired man, he was a mighty warrior. He was captain of the Fianna, the army of the High king. The Fianna was made up of the very best warriors in all Eriu. The people of the clan were very proud of their chief.

Of course, where you find high praise, you also find high scorn. The people of Clan Morna hated Cumhaill and thought their leader should be captain of the Fianna. They tried to use trickery, lies and cunning to dispose of Cumhaill, and hatred grew between the two clans.

Finally, the hatred exploded into war. Clan Bascna was defeated and Cumhaill was killed. Those men who did not die went into hiding. The women of the clan were left to grieve their lost.When Murna, Cumhaill's wife, learned of his death she donned her lcoak and fled into the shadowy forest of Slieve Bloom. She knew that the Morna clan would seek her out and kill her. They would not allow the kin of Cumhaill to survive, in order to ensure that revenge would not be taken.

It was in the depth of the forest, and the depths of her sorrow that she gave birth to a baby boy, whom she named Demna. She feared for his life, should Clan Morna learn of his existance. She took the child to a hidden cottage, in which dwelled two wise women, Bovmall and Lia Luachra. Bovmall was Cumhaill's sister, and Demna's aunt. The child was left in their care, and his mother disappeared.

Demna grew up in the woods, playing with the woods creatures. The squirrels taught him to climb the tall trees, the deer to run, the rabbits to dodge and the fish to swim. The two women sent him to Fiacuil mac Cona, a comrade in arms at one time to his father. He taught Demna to through a spear that never missed, to swing a sword that cut through even the stoutest shield and the use of warrior's skills.

Occasionally the boy wandered to the edge of the forest to play with the children of a small village. Because of the boys light colouring, hair and skin, the children called him Fionn, meanign blond or fair.

All this time Fionn, or Finn, grew up in grave danger. Clan Morna had learned of his existance, and were seeking him, determined to slay him. When Finn became a man he decided it was time to follow his dream, and become captain of the Fianna. He planned to set out for Tara, where the High King dwelt.

First though, there was something he needed, for though he was the swiftest of runners, the highest of leapers, and the best of swimmers, as stong as two men feared nothing and was skilled in every kind of weapon...he had no wisdom. He said his farewells to Bovmall and Lia and set out for the River Boyne, where there dwelt a wise man and poet by the name of Finegas. Finn offered to cook, clean and chop wood in return for learning the man's wisdom. The man agreed and asked for his name. Finn, afraid that Finegas may be hurt if Clan Morna discovered him, gave the man his first name, Demna.

Finegas taught Finn the magick of poetry, the charm of music, the making and solving of riddles.

One day Finn asked his teacher why he sat by the river day after day. Finegas told his student the legend of the Salmon of Knowledge. It had been foretold that Finegas would catch the fish one day. And whosoever ate the Salmon would gain all the knowledge of the world.

Days, weeks and months went by. One day Finn heard his master call him from the river. He rushed out to see what had occured. Finegas was sitting witha basket in his lap, inside lay a large, orange and gold fish. Finegas had caught the Salmon of Knowledge. Finn was delighted and ofered to cook the fish for him.

When the cokked fish was placed before Finegas, the old man looked at Finn and asked if he had eaten of it. Finn explained that a blister had formed and he had burst it with his thumb. The juices had burned his thumb and he had quikly put it in his mouth, therby tasting, but not eating. Finegas smiled at his student and told him two things. He, himself, would never eat the fish, and that he knew that Finn was not Demna, but Finn MacCool.

Finn fell back in surprise. Finegas explained that while it was foretold that he would catch the Salmon, it was also fortold tht only Finn MacCool would eat it. Since Fin had tasted the fish, he must be Finn MacCool. And so Finn ate the Salmon, gaining knowledge with every bite. He know knew why the wind blew and the stars shone. He know had all the skills he would need and set off for Tara."

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In other Celtic tales from both ireland and Scotland I have seen Salmon also percieved as wise fish, also long living too.
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