Irony is when the opposite of what is expected occurs. "The Adventures of Huckleberry Finn" is full of irony. A lot of it occurs in Huck's attitude towards slavery; he feels that slavery is right, and that helping Jim escape is wrong. Now, given the context of the novel, and its setting, that might not be considered unusual. Huck was raised in a slave-holding state where slavery was considered the way of life. Abolitionists at the time were considered "low-down" or immoral. But if you hold Huck's attitude up to the light of our society's expectations, it is ironic. You would expect him to be glad to help Jim-a man with a family only searching after freedom, after all-and to be happy to help him escape and obtain his independence form an immoral system. So, we feel the irony of Huck's attitude. You could find all three examples of irony within this one concept: When Huck runs across Jim on the island, he agrees to help Jim even though people would think he was a "low-down abolitionist"; when Huck hears Jim getting excited about earning money to free his family, Huck experiences anger that he would be so ungrateful to steal "children that belonged to a man I didn't even know; a man that hadn't ever done me no harm"; and when writing a letter to Miss Watson to tell on Jim, Huck tears the paper up and thinks to himself, "All right then, I'll go to hell!"
These three examples demonstrate Huck's rather ironic attitude about slavery and the very noble and kind thing he is doing for Jim in helping him to escape.