"Piggy and Ralph, under the threat of the sky found themselves eager to take a place in this demented but partly secure society. They were glad to...

The irony in this passage is primarily situational.  The opposite happens from what is expected.  The quotation you cite occurs in Chapter 9 when Ralph and Piggy join Jack's group in the feast, the re-enactment of the pig hunt, and ultimately in the murder of Simon.   Part of the irony lies in the characterization of Ralph and Piggy.  These two characters along with Simon are the sanest and most moral characters in the novel....

The irony in this passage is primarily situational.  The opposite happens from what is expected.  The quotation you cite occurs in Chapter 9 when Ralph and Piggy join Jack's group in the feast, the re-enactment of the pig hunt, and ultimately in the murder of Simon.   Part of the irony lies in the characterization of Ralph and Piggy.  These two characters along with Simon are the sanest and most moral characters in the novel. Yet they are "eager" and "glad" to participate in a cruel and barbaric ceremony.  

The theme that is presented here involves the need to belong.  Because of their fear of the storm and the darkness, Piggy and Ralph would rather be part of a society--any society--than be alone. So when they willingly join the group, they take on the characteristics of the group and become savage murderers.  

Further situational occurs in the reversal of order and disorder.  The fact that there is  a type of order in this "demented" society is not expected.  The boys know their parts as they become hunters circling a pig.  For them, this order is a type of security that "hem[s] in the terror."  Ironically, there is no true security in this group.  It can turn on its own and kill cruelly, without mercy. The terror within the group is much more deadly than the storm.  When Simon enters the circle, the hunters including Ralph and Piggy attack him viciously.  Their play-acting escalates into an actual murder.