The sentence that is an example of irony in the excerpt is "I wish to goodness we had a few fools left."
This refers to the use of words with an opposite meaning, which means one person says something but means another.
There is verbal irony in "I wish to goodness we had a few fools left" because Jack first acknowledges his friend being clever and then he criticizes the cleverness. Moreover, likely, Jack does not want more fooled people in the world.
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