What is ironic about the ending?

There are a few ironies here. Ralph was once leader of these boys and now he is alone and hunted. 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.