Is irony ever used to comic effect in The Great Gatsby?
The Great Gatsby doesn't strike this reader as particularly comical, but I can think of one part where Nick's ironic commentary on Myrtle Wilson might produce some comedy. Myrtle, Tom's mistress, is of a much lower class than either Tom or Nick. She lives in the valley of ashes (rather than either of the "Eggs") with her mechanic husband, George. One would, therefore, likely not expect her to behave in a snotty way -- as we might expect of Tom or Daisy. We would more likely expect Myrtle to behave humbly, as a person who is used to less and receiving more might.
However, once she is ensconced in her apartment with Tom and Nick, Myrtle changes her dress, and "With the influence of the dress her personality had also undergone a change." She begins to behave with "impressive hauteur. Her laughter, her gestures, her assertions became more violently affected moment by moment, and as she expanded the room grew smaller around her, until she seemed to be revolving on a noisy, creaking pivot through the smoky air." Such an image, of Myrtle swelling like a balloon in an ever-shrinking room, spinning around and around noisily as she tries to impress everyone around her, is a pretty comical one, and it is ironic because her behavior is so different from what we would expect of a woman in her position.