The Salmon of Knowledge / An Bradán Feasa
This story comes from the Fenian Cycle, one of the four great cycles of Irish mythology. It's one of the oldest origin stories in Irish literature, explaining how Fionn Mac Cumhaill became who he was.
The Salmon of Knowledge holds a special place in my heart. It's a story of how Finnegas, the poet, spent seven years trying to catch a fish that holds all the knowledge in the world, but it's his young apprentice Fionn who ends up with it, by accident.
There's a lot more about how that happens over on my blog.