Ironically what Ralph has wished for does come true. Why is what happens in answer to his wish depressingly ironic?I couldn't find the answer!...

In addition to the irony that the fire on the top of the
mountain, meant for rescue by a plane is out and the mountain is the receptor of a dead
parachutist,  there is the sad irony of the parchutist's body
being

a sign
[that]came down from the world of grown-ups, though at the time there was no child awake
to read it.

Rather than
interpreting the parachutist as representative of the evil and savagery in man that
wages war, Samneric interpret him as the beast when they climb the mountain to stoke the
signal fire that has burned out.  Only Simon, who when he thinks of the beast, has
rise 

before
his inward sight the picture of a human at once heroic and
sick.

An adult has come from
the civilized world, but he is a sign Ralph has not sought.  It represents man's
intrinsic illness as he is a man from battle, and, ironically, Simon is the only one who
can recognize it.