What joke does Angelica play on both Robin Dempsey and Persse?

Angelica sets a date with Persse to reenact a scene from one of her romance poems, "The Eve of St. Agnes," in which a woman undresses while her lover watches, hidden. She gives him her room number, 231. While Persse is hiding, Robin Dempsey comes into the room, disrobes, and gets into bed; when Persse shows himself, he discovers it is actually Dempsey's room. Angelica had set a date with Dempsey as well, to reenact a scene from an Eyetie poem, apparently to pull a joke over on both of them.