Answer:
The biggest irony is that the boys are rescued because of Jack lighting the island on fire.
Step-by-step explanation:
The fire has two additional meanings. As Ralph hides in the jungle, he realizes that Jack has set the jungle on fire to smoke him out. Fire now is a symbol of savagery and destruction - the opposite of civilization. The depth to which the boys have fallen is shocking.
It was not the signal fire that attracted the navy cruiser. Instead it was the forest fire that Jack's tribe set in an extreme gesture of irresponsibility and self-destruction. Ironically and even tragically, it is Jack and not Ralph who is ultimately responsible for the boys' rescue.