This is quite nicely written ...

http://www.hoganstand.com/general/identity/extras/wayoflife/stories/holywell.htm

Haven't read this yet

http://www.treesforlife.org.uk/forest/mythfolk/hazel.html

And from

http://orgs.carleton.edu/Druids/ARDA2/doc/2part6-9.doc

The Salmon is mythically famous for its association with Hazelnuts. The primary story is that of young Fionn (nicknamed Demne) who as a boy was learning bardic skills from Finegas on the rivers of Boyne. Finegas had been patiently fishing for seven years for the Salmon of Knowledge, which had fed on fallen hazelnuts from nine magical (non-descript) hazel trees. The Salmon gained a spot for every hazelnut that it ate. Soon after Fionn's arrival, the fish was caught. Fionn was frying the fish for

Finegas, who was off on a trip, and a boil rose on the fish, he pushed it down with his thumb and burned himself. When he put the thumb in his mouth, the knowledge had all transferred to him. Finegas was, of course, a little disappointed, but Fionn shared the remainder with him and promised another Salmon would come along soon.

Fionn's way of discovering whatever was happening and hidden was always the same. A shallow, oblong dish of pure, pale gold was brought to him. This dish was filled with clear water. Then Fionn would bend his head and stare into the water, and as he stared he would place his thumb in his mouth under his "Tooth of Knowledge," his "wisdom tooth."

The ancient Fianna warband had many special fighting techniques that would be the envy of any martial art movie. One of the most famous besides Caber tossing (to throw bridges for chariots over rivers) was the "Hero's Salmon Leap," which consisted of leaping on top of a standing shield and leaping high up for a "smackdown" on your opponent.

There are also references in Goidelic lore to Salmon being kept in wells (near Hazel tree orchards) for oracular consultation. In the ancient text "Cormac's Vision" the hero sees a royal fortress with four houses in it, and a "bright well" surrounded by ancient hazels. In the well were five salmon, which ate the nuts as they dropped. In the palace, Cormac meets Manannan the sea-god who reveals the Land of Promise to him and presents him with a magic cup and branch. www.enya.org/stories/story06.htm. Later on, Cormac MacArt, king of Ireland in 266 AD, died at Cleiteach, the bone of a salmon sticking in his throat, on account of the siabhradh genii which Maelgenn, the Druid, incited at him, after Cormac had turned against the Druids, on account of his adoration of God in preference to them. So beware Druids bearing dinner. In Christian monastic communities, there were often salmon ponds for eating. In May 11, 1113AD; "A salmon was caught at Cluain-mic-Nois this year, which was twelve feet in length, twelve hands in breadth without being split, and three hands and two fingers was the length of the fin of its neck." (This is the site of the ruins of the monastery of Clonmacnoise in County Offaly on the River Shannon below Lough Ree and above Portumna, which is in County Roscommon. The arms of Co. Meath incorporate the salmon also.) Christ is also known as in t-eo sénta cas corcra, "The Blessed Curled Purple Salmon." This seems strange, considering that grown salmon prefer salt-water, but I suppose they can remain in fresh-water if not given a choice;