Life is rough for the Younger family, and Hansberry's use of somber tone is appropriate to that. At the same time, however, she injects a heavy dose of irony and sarcasm. Did you notice how Hansberry writes "Drily" in a lot of the line directions? (Wait, is that just "dryly" spelled a different way? Yes, it is.) The Youngers have a bite in how they talk; there's a fun tongue-in-cheek kind of feel, especially in Walter and Beneatha's sibling chats.
One of the single most ironic moments in the play, however, might be when Mr. Lindner explains that the people he represents have worked hard to achieve their dreams. In that single scene, the characters don't notice the irony so much as the audience does. It's Hansberry at her finest – exposing how the American Dream can ring hollow for black Americans.