What was ironic about the punishment given to the Cunningham gang in To Kill a Mockingbird?

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.