This is a good question. There are two ironies here, but we before we look at them, let me give you the quote.
Today Aunt Alexandra and her missionary circle were fighting the good fight all over the house. From the kitchen, I heard Mrs. Grace Merriweather giving a report in the livingroom on the squalid lives of the Mrunas, it sounded like to me. They put the women out in huts when their time came, whatever that was; they had no sense of family—I knew that’d distress Aunty—they subjected children to terrible ordeals when they were thirteen; they were crawling with yaws and earworms, they chewed up and spat out the bark of a tree into a communal pot and then got drunk on it.
Immediately thereafter, the ladies adjourned for refreshments.
The first irony comes from the fact that the women of the missionary circle were discussing the plight of the poor Mrunas and the diligent labor of Mr. Everett Grimes, whose name comes up later in the chapter. The odd juxtaposition of the the poverty and difficult lives of the Mrunas tribe and the wealthy ladies of the missionary circle is a microcosm of what is going on in Maycomb. In Maycomb there are the poor blacks who are being mistreated and the people of "high society," who are only Christian in name and not in character. The irony is that most of the women of the missionary circle do not see the injustices in their own backyard.
Second, on a more superficial level, the talk of chewing and spitting in a communal pot and then drinking it is extremely unappetizing. So, for the women to get refreshments is humorous, to say the least.