Irony/Ironic

irony/ironic: the contrast between what is stated explicitly and what is really meant; the difference between what appears to be and what is actually true. Also used to define tragic contrast between the aspirations of human beings and the dark elements of life that frustrate them; in addition, when used to describe to view of humanity in which limitations and posturings are seen as debasing and ridiculous, there is in the ironic view an element of mockery. Irony may be verbal, situational, or dramatic

"And his Aunt Polly she said Tom was right about old Miss Watson setting Jim free in her will; and so, sure enough, Tom Sawyer had gone and took all that trouble and bother to set a free nigger free! and I couldn't ever understand, before, until that minute and that talk, how he could help a body set a nigger free, with his bringing-up." 
                                                                              -Adventures of Huckleberry Finn
                                                                                          by Mark Twain

Twain informs the reader of Jim's freedom after he has been freed. Jim was set free by his mistress, but held captive and freed  again, in a rather round-a-bout manner, by the only boy who knew of his original freedom. This is an example of situational irony. The irony lies in the fact that the characters went to quite a bit of trouble, including being shot and eating sawdust, only to free someone who was already legally free. Twain uses this to characterize Tom and emphasize his "hopeless romantic" side. Tom wanted the big daring escape, creating a rope ladder for an escape from a one-story shack, and digging a whole under the shack to help Jim escape, all the while knowing that Jim was perfectly free. Twain is showing that Tom is not particularly sensible, but is imaginative, and in a way, a leader.