She appears primarily in the Irish tale of Serglige Con Culainn (The Wasting Sickness of Cú Chulainn), where she is the daughter of Áed Abrat. She appears first in the form of a sea bird, then as an otherworldly woman who inflicts the story's eponymous sickness on Cú Chulainn. 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.
This Lí Ban may be identical in origin with the sister of Fand, or may be an independent tradition. In Irish Christian legend, Lí Ban was a woman most prominently mentioned in the Annals of the Four Masters, in which she is the daughter of one Eochaid. She turned into a mermaid when a spring burst under her house to form Lough Neagh (Loch nEchach in Old Irish, 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.