In Ch.12 of Lord of the Flies, Ralph says to himself, "You'll get back." Explain why this is ironic.
In Ch.12, Ralph says to himself, "you'll get back." Compare this with what Simon said to Ralph about returning early in Ch.7. Explain the irony of his no in Ch.12.
Simon tells Ralph "You'll get back" and "I just think you'll get back all right" early in Chapter 7. This is an example of foreshadowing, predicting Ralph's frantic escape from the tribe in Chapter 12, which drives him into the arms of his rescuer. In this sense, it predicts both deliverance from the island, and deliverance from the tribe. The repetition of the phrase emphasizes it so that, when Ralph uses the same phrase in Chapter 12, it recalls Simon's prediction to us.
Ralph's usage of the phrase occurs at a key point in the story, as he stares into the eyes of one of the savage boys pursuing him. So the phrase is meant to lead us through the climax of the final pursuit by giving us a sense of hope and expectation that Ralph really will "make it back all right".
This sense is rendered even more powerful since the phrase was originally uttered by Simon. Simon's earlier understanding that the beast was not all that it seemed and his death at the hands of the tribe lend his words an otherworldly wisdom (often seen with characters nearing their death - can be interpreted as them nearing the wisdom of eternal life).
For the "no" in Chapter 12, I'm assuming you mean the part where he says "No. They're not as bad as that. It was an accident". Since we (and Ralph) know full well that the tribe made a savage attempt on his life, there is a discord between his words and the actual situation which highlights the horror of the tribes' actions. Since the discord is in Ralph's words, this is verbal irony.