As told in the Dindsenchas, Boann created the Boyne. Though forbidden to by her husband, Nechtan, Boann approached the magical Well of Segais (also known as the Connla's Well), which was surrounded by hazels. Hazelnuts were known to fall into the Well, where they were eaten by the speckled salmon (who, along with hazelnuts, also embody and represent wisdom in Irish mythology). Boann challenged the power of the well by walking around it widdershins; this caused the waters to surge up violently and rush down to the sea, creating the Boyne. In this catastrophe, she was swept along in the rushing waters, and lost an arm, leg and eye, and ultimately her life, in the flood. The poem equates her with famous rivers in other countries, including the River Severn, Tiber, Jordan River, Tigris and Euphrates.
She also appears in Táin Bó Fraích as the maternal aunt and protector of the mortal Fráech.
Modern-day commentators and modern paganism sometimes identify Boann with the goddess Brigid or believe Boann to be Brigid's mother; however there are no Celtic sources that describe her as such. It is also speculated by some modern writers that, as the more well-known goddess, and later saint, the legends of numerous "minor" goddesses with similar associations may have over time been incorporated into the symbology, worship and tales of Brigid.