Ralph tells the boys in the beginning of the novel that his father is a Naval officer. The military is symbolic of adults, of society. When the boys are stranded on the island, due to their inherent evil nature (which Golding believes we all have in our hearts), they resort to the "beasts" that they are, in their hearts. All mankind, left to his own devices, resorts to evil, Golding seems to be implying. It is not society that it is evil, it is man. The only thing that keeps this evil in check is society, rules, or on a higher level, morality and God. Golding attempts to show this by having a group of innocent children, uncorrupted by society, turn evil when they are not under the constraints of society. The fact that the children can turn into beasts shows that evil is within us all.
Therefore, it is irony when a symbol of that very society shows up to rescue the boys. As soon as the Naval officer appears, the boys immediately become children again and start crying. Plus, the appearance of the officer with his ship in the distance proves that the war is still going on, another irony because the boys are removed from society's war, but they have been engaged in another war - a battle that takes place on a spiritual level.
It all depends on how you view the military and their role in the conflict that plays itself out on the island, mainly that there is evil inside of all of us and given the right circumstances, we are quick to let it take control.
The officer acts as though the boys ought to have behaved better as he stares out at his trim cruiser, bringer of death and destruction to many... The boys are in the midst of their conflict while a war rages around them, thus the parachuting dead body. But the irony, in my mind, is that these boys are being "rescued" by a society that engages in almost constant violence but likes to make it look civilized by making the participants wear uniforms and by justifying their wars.
I think that what you would probably want to say is that it is ironic to have the military (an organization whose purpose is, ultimately, to kill people) be the ones who rescue the boys from killing each other. The problem that the boys have is that they are unable to keep themselves from being violent, yet their salvation comes in the shape of a violent institution.
I do not know that I agree with this, since the military is also really supposed to be about protecting our civilization from people who want to act like savages. In the military, violence is only used at the "right" times and for the "right" reasons. So I don't really think this is ironic.
The irony lies in the fact that they would never have been evacuating England and crashed on this island if it weren't for warring militaries. Even as Ralph longs for a sign from the outside world, the sign he misses is a pilot shot down thousands of feet overhead.
Ralph mentions his father the Naval officer in the first chapter, but Piggy reminds him that no one knows where they've landed; so even the military can't find them now.