Irish dinnshenchas, the lore explaining the names of places, tells how the River Boyne came to exist. At the Well of Segais, also called Connla's Well, grew the nine hazels of wisdom, whose nuts fell into the water and were eaten by the salmon of knowledge that swam there, the source of poetic and prophetic wisdom in Irish tradition. Only Nechtan, Boann's husband, and his cupbearers were permitted to approach the well, and any other who circled it was said to lose an eye, a hand, or their life to its power.
Boann defied the prohibition, whether from curiosity, pride, or a wish to test the well's power against her own, and walked around it three times. The well rose up against her transgression in three great waves, taking an eye, a hand, and a leg from her before she fled toward the sea, and the flood that pursued her carved the course of the River Boyne across the Irish midlands. In this telling the river is not simply named for a goddess but is the physical trace of her punishment, a landscape myth in which geography records a moral event.
Other versions soften the injury or omit it, framing the flood as Boann's own act of claiming a river as her domain rather than a penalty imposed on her, and the differing tellings are usually read as evidence of the story circulating in more than one form even within the medieval manuscript tradition that preserves it, rather than as a single fixed original since corrupted.