In chapter 5, how is the wish for a "sign from the world of grownups" ironic?
Passage from Lord of the Flies by William Golding, Chapter 5: "If only they could get a message to us," cried Ralph..."If only they could send us something grownup....a sign or something."
Chapter 5 may introduce the sign, but doesn't fully develop the idea (that will come in later chapters.)
The irony behind the "sign from the world of grownups" is that the sign is, indeed, a grownup. At least a dead one. The boys discover that a pilot is hanging by his parachute from a tree just as dead as a doornail. That's the irony. They get a sign and it is literally a grownup.
Also ironic is the destabilizing influence that this "sign" has, because at first it is associated with the concept of the "Beast." When Samneric relight the fire and illuminate the swinging man they assume it is the beast come to drag them off. The man becomes part of the destabilizing force that is the beast.
I am not sure what to make of your question about a "message." As it is dead it doesn't tell them anything, nor do they know what it is. The only message they get is that the Beast is, indeed, a real force on the island, furthering their break with reality.
Chapter Five of Lord of the Flies finds Ralph frustrated that he cannot be the leader that he would like to be. And, as the boys regress into discord and disorder, Ralph understands the urgency for an assembly to re-establish order; however, at the same time he realizes that he is unable to think as logically as Piggy, who, unfortunately does not have the other qualities requisite for a chief.
When the assembly is called about the boys' fear of the beast, Jack exerts his physical prowess as a hunter as reason for their not being a beast: "I'd have seen it...there is no beast in the forest." Then, Simon stands and becomes
inarticulate in his effort to express mankind's essential illness.
But, when he tries to explain, the boys' laugh; after this incident Jack seeks to usurp control by declaring savagely,
Bollocks to the rules! We're strong--we hunt! If there's a beast, we'll hunt it down! We'll close in and beat and beat and beat--!"
Clearly, Jack disrupts the rationality of the group, creating fear in the others along with chaos. It is at this point that Ralph desperately wishes there could be a message from the adults telling the boys what to do. Ironically, Ralph perceives the adult society as rational and capable of logical decisions, while, in fact, it is the adult world which is at war and it is the adult world that has caused the airplane on which the boys were passengers to crash and place them in the predicament in which they now exist. Thus, the adult world is no more in control than the boys themselves.