The irony is that Ralph has continually sought to remind the boys of the need to keep a signal fire lit in order to stand a chance of being rescued. At this point in the novel when Jack has given himself over completely to savagery and dismissed all thoughts of rescue from his mind. Jack's only intention in lighting the fire was to drive Ralph out of hiding with the intention of then killing him. That this murder motivated fire, lit by a person with no thought of rescue, should actual serve as the mechanism which rescues Ralph from Jack's intentions and indeed rescues all the remaining boys from the island and Jack's tyranny is without doubt ironic.