Fionn MacCumbhaill was a warrior chieftain in Ireland in the 3rd century AD. He lead a clan of warriors called Na Fianna and their story is chronicled in the Fenian Cycle. Fionn was regarded as an extremely wise and clever warrior. One of the stories of how he gained his great wisdom concerns the Salmon of Knowledge.

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As a youth, Fionn was sent to live with a wise old man named Finnegas, who agreed to train Fionn and teach him everything he knew. Finnegas had learned from the Druids that in the River Boyne lived a magical salmon known as the Salmon of Knowledge. Anyone who ate the flesh of the salmon would gain all its wisdom.

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One day Finnegas caught the magic salmon. He told Fionn to cook it, and warned him not to eat it. As he was roasting the salmon, a blister appeared on its flesh. Fionn burst the blister with his thumb, burning himself, and stuck his thumb in his mouth to ease the pain. As soon as Finnegas saw Fionn, he recognised from the wisdom in his eyes that he must have eaten the flesh of the salmon.