Lí Ban was a sister of the sea goddess Fand ("the Pearl of Beauty") and wife to Labraid Luathlam ar Cledeb ("Labra of the Swift Hand on the Sword"), the ruler of Magh Mell. She appears in the Irish tale of Serglige Con Culainn (The Wasting Sickness of Cú Chulainn), first in the form of a sea bird, then as an avenging goddess. In the story Lí Ban acts as messenger and mediator; she and Cú Chulainn's charioteer Láeg work together to see that Cú Chulainn is healed in exchange for his aid in Fand's battle in the Otherworld.
In Irish Christian legend, another Lí Ban was a woman who turned into a mermaid when a spring burst under her house to form Lough Neagh (in what is now Northern Ireland). She and her dog (who in the flood became an otter) lived trapped under the lake for 300 years, until she was fished out by a group of monks in the time of Saint Comgall. The mermaid was brought to shore and baptized Muirgen ("sea-born")—thereby sacrificing her immortality for a Christian soul.