After Archer has accepted that he is in love with Ellen (and they have started trying to plan secret alone time together), Ellen’s aunt Medora approaches him to ask him for his assistance in reuniting Ellen with her husband. This is an example of dramatic irony, because Archer and readers know that he is the last person who wants to see Ellen go back to Europe to be with her cruel husband.
Archer’s inner reaction to Medora’s request captures the irony of this moment:
He would have laughed if any one had foretold to him that his first sight of poor Medora Manson would have been in the guise of a messenger of Satan; but he was in no mood for laughing now, and she seemed to him to come straight out of the hell from which Ellen Olenska had just escaped.
The irony of Medora’s request is so palpable that Archer wants to laugh. He even uses verbal irony, calling her “a messenger of Satan” (sarcastically comparing Ellen’s brutish husband to Satan). Of course, given that Archer is bound by the rules of elite New York society, he cannot say how he actually feels. He does make his displeasure with this idea clear—telling Medora that he does not think this is a good idea—but doesn’t dare to tell her about his romantic feelings for Ellen or his desperation for her to stay in New York.