As the story progresses, Billy becomes increasingly suspicious of Mrs. Muldoon. He begins to believe that she is murdering her guests and stuffing them into the animals. Billy eventually tries to escape, but Mrs. Muldoon catches him and locks him in her attic.
The ironic twist in the story comes at the end when it is revealed that Billy was wrong about Mrs. Muldoon. She is not a murderer, but rather a kind and caring woman who is trying to help her guests. The elderly people in the house are not dead, but rather are in a state of suspended animation. Mrs. Muldoon is using her taxidermy skills to preserve their bodies so that they can live forever.
The ending of the story is ironic because it completely subverts the reader's expectations. The reader has been led to believe that Mrs. Muldoon is a murderer, but it turns out that she is actually a good person. This twist ending adds an extra layer of suspense and horror to the story.