Observing other elderly people who come to the park "Sunday after Sunday," Miss Brill criticizes and dismisses them, even though she has much in common with them herself. This is a moment of dramatic irony:
"They were odd, silent, nearly all old, and from the way they stared they looked as though they’d just come from dark little rooms or even – even cupboards!"
Dramatic irony occurs when the reader knows something the characters don't. In this case, Miss Brill seems unaware that in criticizing her neighbors, she's essentially describing herself. Like them, Miss Brill is elderly; she makes a ritual of coming to the park every week; and she's silent because she has no one to talk to.
The reader can already intuit that Miss Brill is more similar to her neighbors than she knows, but later developments in the story make this reality more explicit. When the young couple sits down near Miss Brill, the man describes her as a "silly old mug," a version of the description (if a rather more uncouth one) that Miss Brill applies to her neighbors. At the end of the story, Mansfield describes Miss Brill's apartment as a "little dark room," mirroring the language used in this passage and strengthening the comparison even more.
The dramatic irony of this moment serves to emphasize Miss Brill's tendency towards delusion and her reluctance to acknowledge her own place in the world. Through Miss Brill's tendency to criticize those most like her, her unstated self-loathing becomes clear.
