Just had a chance to open the book back up, and from the note on the text by Christopher Tolkien, corrigan is a Breton word for fairy. From the introduction by Verlyn Flieger, she expands a bit more to say:

In the Lay she represents a particular subset of this type [of Celtic seductive otherworldly female figures] called a corrigan, malevolent, sometimes seductive, whose dangerous attraction embodies both the lure and terror, the ‘fear of the beautiful fay’…