Eilis is introduced in the book Daughter of the Forest and is described as being "a quiet girl, plump and rosy-cheeked". She is fifteen when she first meets Liam and has blue eyes, long eyelashes and a "mane of hair the same startling hue as her father" (red). She appears to be rather shy but grows to love her betrothed, Liam. They announce their engagement and celebrate at the house of Sevenwaters, only to be interrupted by the arrival of the captured Simon.
When Liam and his brothers are turned into swans, the betrothal is dissolved. Although Eilis loved Liam, she marries Eamonn of the Marshes out of duty to her father. She produces his heir, Eamonn, who plays a vital role in both Son of the Shadows and Child of the Prophecy. She dies in childbirth when her daughter Aisling is born and, although it is not stated precisely how, her husband Eamonn of the Marshes is removed from power and alliance for his treachery and collusion with Richard of Northwoods.
Eilis is mentioned by her daughter Aisling at a ceremony of remembering the dead at Samhain. Aisling later names her sixth daughter Eilis to honour the memory of her mother.