It is more the irony of the entire circumstance involving the fire there. Early in the book, the fire was two things: it was a way to signal for a rescue, and it was a tool to cook food and keep them warm. In both cases, it was a marker of civilization. However, by Chapter 12, things have reversed. Fire is now a sign of savagery, as the boys are trying to burn Ralph out. Rather than helping them stay civilized, it burns down the shelters (destroys visible signs of community). Then, at their lowest point, their rescuers show up. Talk about irony! They aren't found when they're trying to get home, but rather when they've given up on being civilized.
The imagery of fire is an important symbol in the book. At first the boys need fire for survival, as their only chance for rescue is to create a signal fire. They use Piggy's glasses to make the fire. This shows the ingenuity of the boys and the importance of fire for future survival. In this regard, fire is also a symbol of order and civilization. Most importantly, they control the fire.
Then fire is also needed to cook food. This, too, is a symbol of survival, even though the meaning here is more ambiguous. However, in the end of the book - chapter 12 - 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.
The saving grace of fire though is that the destructive blaze in the jungle ironically functions as an enormous signal fire, which a ship has seen. This causes a rescue to take place.
the irony of the fire is the fact that jack tried to kill ralph or smoke him out when in fact the fire ended up attracting attention to the british ship saving them all