I would contend that irony is the rhetorical device used in this statement from Dr. Martin Luther King Jr.’s famous speech “I Have a Dream.” He is saying that he dreams (so far what he dreams of has not come true) of one day when the United States will rise up (both literally and metaphorically) and once and for all live according to the true meaning of its creed - a flagrant reference to the Declaration of Independence, which states that one of the truths, or creeds, that Americans hold is "that all men are created equal." Dr. King even refers to this part of the Declaration in his own speech. What he is here emphasizing and reminding his audience about, with a certain degree of irony, is that, so far, the nation as a whole has not held that truth as self-evident, so he has a dream that one day it will finally be true to its word and live, and let live, according to that inalienable, and inherently American, principle.