This is a poem that Tolkien wrote about a noble couple that wanted to start a family, and the corrigan that meddled with their dreams.
That’s a great question, and one I should have probably anticipated, so apologies! In the book, the editor (Verlyn Flieger) gives the context there. I don’t have the book in front of me right now, so I’ll have to paraphrase, but if I’m appealing to brevity of words: it’s a witch.
More specifically, it’s a fey creature that usually lives in a magical part of the woods and lures men to her. Her intent varies, it’s usually one of either a) trying to get the man to leave a recently-wedded wife in order to marry the corrigan, or b) trying to get a recently-married man to promise a future child or children. She can shape-change from an ugly form into a more beautiful one to help trick her victim.
I read the first example as the corrigan attempting to get the wife to leave the husband for her, but I probably shouldn’t have expected lesbianism in any Tolkien text.
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’…
