According to dictionary.com, "irony (dramatic irony) is a literary techinique by which the full significance of a character's words or actions are clear to the audience or reader although unknown to the character."
In Chapter 26, Scout has started the third grade, and as an assignment, her teacher, Miss Gates, gives a current events assignment about Hitler. Miss Gates lectures to the class about the persecution of the Jews by Hitler and how atrocious it is to live in a dictatorship. Scout doesn't understand the hypocrisy Miss Gates shows by being sympathetic to the Jews but not the blacks who are persecuted in Maycomb. She is beginning to see the double standard many whites show in town. She asks Atticus about it, but doesn't get very far, so she goes to Jem to try to get an explanation. She asks Jem, "How can you hate Hitler so bad an' then turn around and be ugly about folks right at home?" This is a good lesson for Scout as she is beginning to see the hypocrisy and discrimination so prevelent in Maycomb. This shows Scout's growth as a person who is consciously aware of the differences in treatment of people.
As a reader, we understand the dramatic irony of the hypocritical views of Miss Gates as well as the other people of Maycomb. While Scout is figuring it all out, we as readers have already gotten the message Harper Lee is attempting to send.