What is ironic about miss prism's expressing concern for jack and admiration for his devotion to his brother? what kind of irony is this? this is a question from "The Importance of Being Earnest" and I'm having trouble understanding what exactly it means.

This is the story entitled "The Importance of Being Earnest".

Miss Prism's expressing concern for Jack and admiration for his devotion to his brother are ironic because she knows from the start that Jack’s pretences of having a brother are eventually real. He then finds out that Algernon is his younger brother upon questioning Miss Prism at the last part of the story.

The story is an example of situation irony because the audience did not expect that Jack has a brother and that it will be Algernon, as opposite to their expected ending.