There were many different descriptions for the definition of irony, all meaning essentially the same thing, but my favorite wording was from grammar.about.com. The use of words to convey the opposite of their literal meaning; a statement or situation where the meaning is contradicted by the appearance or presentation of the idea.
Michael Moore uses irony masterfully in this film with his voice over editorials throughout the film and with the interview of the couple at the Great Gatsby party focusing on what is supposed to be good about Flint. The interview made them seem really out touch with reality
It was also very ironic that he was from the town of Flint and he knew what it had been compared to what it had become. That made it a perfect documentary because it made me the viewer who has never been to Flint, understand what was happening there, or at least Moore's view of what was happening there..
Moore also used his interview with Ms. Michigan during a parade that looked very depressing in the background with boarded up stores and less than excited attendees compared to her going on and winning the Ms. USA title as a kind of glamor versus depression. I also found it amazing that they put as much money as they did into the vacation and visitors and conventions department as a last ditch effort to get people to come to Flint on vacation or to bring conventions there.
Another scene that stuck with me was the optimism at the GM Christmas eve party compared to the fact that people were getting evicted from there home and the nonchalant attitude that the CEO had when Moore was finally able to ask him a few questions. Irony was used by playing happy Christmas music while touring the boarded up depressing looking town of Flint at the end of the film. Moore used the rhetorical device of irony in this film to show the viewer from his opinion that the upper class of people were out of touch with what was going on throughout the film.