The trope of irony is a substitution of a term that states the opposite of the intended meaning. It corresponds to the topoi of contraries and contradictories. Grammatically, the term "irony" may be applied to situations that seem contradictory; Dramaturgically, it may be applied to unexpected plot developments or things known to the audience but not to the characters. All of these might figure in the play of irony in public discourse. However here we are mainly referring to its specific definition as a rhetorical trope.
Lanham: "Implying a meaning opposite to the literal meaning" (92).
Corbett: "Use of a word in such a way as to convey a meaning opposite to the literal meaning of the word."
The greek term εἰρωνεία is also translated as "dissimulation." The term invites infinite regress. Here we are concerned with verbal irony, and the benefits of taking the most literal signification of irony as a trope: emphasizing a "truth" by stating its opposite.
In Frederick Douglass' speech to the American Antislavery Society, 9 May 1848, titled editorially "We Have Decided to Stay," he characterizes a debate between the advocates of immediate abolition of slavery and their opponents:
"For seventeen years, Mr. Chairman, the Abolitionists of the United States have been encountering obloquy, scorn, and opposition of the most furious character, for uttering,--what? Their conviction that a man is a man,--that every man belongs to himself and to no one else. In propagating this idea, this simple proposition, we have met with all sorts of opposition, and with all sorts of arguments drawn from the Bible, from the Constitution, and from philosophy, till at length many have arrived at the sage conclusion that a man is something else than a man, and that he has not the rights of a man." [2.1-2].
The literal key to the verbal irony here is the term "sage" at 2.2. By this point it is apparent that "sage", or "wise", means its opposite, "unwise".