An example of irony in The Scarlet Pimpernel is that Marguerite confides in her husband that her brother is in trouble for being accused of assisting the mysterious Pimpernel, and he turns out to the be the Pimpernel.
Irony is when something happens that is the opposite of what is expected.
The baronet Sir Percy Blakeney has been separated from his wife Marguerite ever since she caused the execution of the sons of the Marquis. When her brother Armand is in danger, Marguerite decides to go to Percy for help. He finds her request for help ironic.
[The] murderous dog of the revolution is turning upon the very hands that fed it? . . . (ch 16)
Although Percy blames his wife for her role in the Marquis’s sons deaths, he realizes that he needs to protect Armand. After all, Armand has been accused of being in the League of the Scarlet Pimpernel.
When Marguerite finds the ring in her office, she realizes that her husband is the Scarlet Pimpernel.
Oh! how could she have been so blind? She understood it all now—all at once . . . that part he played—the mask he wore . . . in order to throw dust in everybody's eyes. (ch 19)
Now Marguerite realizes that she should have known all along. She is concerned about her brother’s safety and her husband’s.