Find three or more examples of irony in the ending of the novel.
One example of irony is that the boys on the island are being saved from their "battle" by a naval officer, who believes the boys are acting rather childish. This is ironic because it can be argued that the officer is actually doing virtually the same thing as the boys are by engaging in his own war, but the officer simply does not realize it. Perhaps it's supposed to seem more "accepted" that adults can and do wage war on each other, but these same adults condemn the similar behaviors of children.
Another example of irony is that the fire that was meant to aid in catching and capturing Ralph is what actually saved the boys on the island; in other words, Jack was basically against the signal fire throughout most of the novel so that he and his hunters could act like savages and eat heartily, but Jack and his hunters were the ones who created this "signal fire" in the end.
Finally, Jack attempts to capture Ralph, who arguably could be the last living "symbol" of civilization. Jack doesn't realize, however, that his idea of setting the island on fire is probably going to kill them all.
The enotes link provides a pretty thorough description of each of these, so check that page out if you still have questions or need further clarification.