What is the irony in that the navy cutter was attracted to the island by the fire that Jack built to “smoke out” Ralph

Jack lights the fire to smoke Ralph out. In doing this Jack destroys the island. So, even if Ralph had been killed, the island would have been useless. It is also ironic that this once Eden like setting is turned into a hellish inferno. The biggest irony is, of course, that the boys are rescued because of Jack lighting the island on fire. This is actually a device called a deus ex machina or God in the machine. It is an abrupt ending where a God-like (the naval officer) entity ends the action. It is still nevertheless ironic that Jack's final act of mass destruction ends up leading to the boys' rescue.