From which fish did Fionn mac Cumhaill gain all knowledge?

Fionn mac Cumhaill gained all knowledge from the Salmon of Knowledge.

In the Fenian Cycle tale The Boyhood Deeds of Fionn, the salmon acquired supernatural wisdom after eating nine hazelnuts that fell into the Well of Wisdom. The poet Finn Eces, also called Finegas, spent seven years seeking the fish because whoever ate it would receive its knowledge.

When Finn Eces finally caught the salmon, he gave it to the young Fionn to cook, warning him not to eat any. While turning the fish, Fionn burned his thumb on a drop of hot fat. He instinctively put the thumb in his mouth, unknowingly receiving the salmon’s wisdom. Finn Eces then understood what had happened and allowed Fionn to eat the rest.

The knowledge remained available to Fionn whenever he bit his thumb. This detail explains a recurring feature of his legend: he is not merely a warrior, but also a poet, seer and wise leader of the Fianna. The fish is sometimes known by its Irish name, Bradán Feasa.