What is ironic about the scene where a mob is trying to kill Tom Robinson?
The irony of this scene is that a child proves to be more brave and more wise than a group of grown men. In this scene, a mob comes to threaten Atticus. Lacking a leader, the mob has hidden their identities in the dark and is relying on the strength of numbers to make up for their lack of moral strength. None of those men would be willing to attack Atticus in daylight, alone. But in the dark, with other cowardly men, they feel bolder and more willing to commit violence.
However, this mob is shamed and stopped in their tracks by the precocious Scout. She recognizes Walter Cunningham's father, and has no sense of danger for herself or her brother. She simply sees someone she recognizes, & honestly wants to know why he's there. Her innocence (as revealed by her lack of understanding of "entailments") as well as her outgoing personality force Mr Cunningham to remember that he is an individual, with his own problems and identity. Upon realizing this, he steps out from the crowd & tells everyone else to go home. Thus, he too shows some bravery by breaking from the pack. Yet the true irony is that the children prove courageous in the face of threats, while the adults act irrationally & savagely.
The answer below is wonderful, but another irony of this situation is that among these men are men who are considered to be good upstanding citizens of Maycomb. Walter Cunningham's father is there as are many other citizens of Maycomb that Scout recognizes.
These are men, who on any other day, they would have considered the pillars of the community for their honesty and their business dealings, but here they are now in a lynch mob ready to kill someone. These upstanding citizens who would, on any other occasion, care for Scout and Jem now have murder in their hearts. How do you rationalize these people who you thought you knew to this mob that is now in front of you? Are they good men or are they angry men filled with hate? How does this play in the mind of Jem? Better yet, how does it play in the mind of Scout who eventually stands up to them and shames them into going back home?