According to the legend, the river Boyne was created by Bóann, a goddess of the Other world.

Her husband Nechtan, alongside his three cup bearers, were the only people who were able to visit Segais well (also known as the well of knowledge). Nine hazel trees that encircles the well would drop their nuts, which contained all the wisdom, knowledge and inspiration of the world,  into the water of the well, where it was then eaten by the salmon that swam there. It was said that whoever either drank the water or ate the salmon or the nuts would then gain all that wisdom, knowledge and inspiration for themselves.

Bóann was forbidden by her husband to approach the well, but through her desire for what the well held, she challenged its power by sneaking up to the water and walking counter-clockwise around it, against the sun. As a result of her defiance, the well rose up in a raging torrent. She had seen every moment, every piece of knowledge that had passed and that would come to be. But it was too late. She was swept away by the waves that the well had let loose from its walls, the water tearing away her arms, her legs, and every part of her until she was completely taken by the flood. The waters raged through the otherworld and into ours, creating the river that we now know as the Boyne.