What was ironic about the punishment given to the Cunningham gang in To Kill a Mockingbird?
I assume you are referring to the group of boys that young Arthur (Boo) Radley was hanging out with when he got into trouble in Chapter 1 of To Kill a Mockingbird. Boo "became acquainted with some of the Cunninghams from Old Sarum," and they were eventually charged with disorderly conduct, disturbing the peace, assault and battery and "using abusive and profane language in the presence and hearing of a female." The boys were sent to the state industrial school--all but Boo, that is. Boo's father refused to allow his son to accept this disgraceful fate, so he assured the judge that if released, Arthur would give the town no more trouble. The irony of the situation is that the poor Cunningham boys "attended the industrial school and received the best secondary education to be had in the state." One of them even advanced to the engineering school at Auburn University. Young Arthur, probably the brightest of the bunch, was relegated to seclusion within the Radley House.
I assume that you are talking about what is done to the Cunningham "gang" that Boo Radley hung out with -- the ones that we see in Chapter 1.
If so, I suppose you can say this is ironic because they are not in any way truly punished. They are sent to industrial school, which Scout says is no prison and no shame. They get the best secondary education possible and at least one of them goes on to really make something of himself. (In contrast to what is done to Boo.)
I don't know that this is ironic myself -- it seems like what should happen to young offenders -- they should be sent someplace where they can make something of their lives.
But I suppose you can say it's ironic because you expect punishment to be a bad thing so this is the opposite of what you expect.
You can also say it's ironic for Boo, but in a really sad way because his life gets totally ruined while theirs are helped.