One great example of dramatic irony, which is of course when one character does not know something in the play that other characters and the audience do, is in Act II when George, going out with Beneatha, says "Good night, Prometheus" to Walter. George is making an allusion to Prometheus the Titan, who was famous for teaching man how to make fire and was punished by being chained to a rock and having an eagle peck out his liver every day.
Of course, Walter does not understand the allusion and the sarcastic comment George is making. Prometheus brought something incredibly valuable and worthy for mankind. All Walter wants to bring is a liquor store, which doesn't really compare. Note the way in which this allusion allows George to show off his learning and the way that following the example of the white man places him at an advantage over Walter, who is ignorant of this name and believes that George just made it up.
As was mentioned in the previous post, dramatic irony is when the audience knows more about a situation than a character or actor. In Act One, Scene 2, Beneatha criticizes her mother for not knowing more about African culture. Lena proceeds to confuse the country of Liberia for Nigeria when Beneatha tells her about Joseph Asagai's home country. Lena also admits to her daughter that she donates money to her church to help save Africans from their "heathenism." Beneatha responds to her mother's ignorance by telling her, "I'm afraid they need more salvation from the British and the French" (Hansberry, 14). Beneatha then begs her mother not to ask Joseph any ignorant questions about his heritage when he comes over to visit.
When Joseph Asagai arrives, Lena pretends to know about African culture by quoting her daughter. She tells Joseph,
"I think it's so sad the way our American Negroes don't know nothing about Africa 'cept Tarzan and all that. And all that money they pour into these churches when they ought to be helping you people over there drive out them French and Englishmen done taken away your land" (Hansberry, 16).
Joseph Asagai is impressed with Lena's knowledge and conviction concerning the plight of oppressed Africans. This is an example of dramatic irony because the audience is aware that Lena is simply quoting what her daughter said about Africans. Joseph Asagai is unaware of Lena's ignorance.