There are several instances of irony. First, the ladies are a missionary circle--ladies from the church that get together to aid in church activities, plan events and other helpful activities. Instead of doing that, they mostly sit around and gossip and talk poorly of others in the community--not a very "holy" activity.
The ladies begin talking about J. Grimes Everett and how he's helping the poor Mruna tribe over in Africa. The women are supportive of his efforts helping this tribe (whose members are black) halfway across the world, but the ladies think it's disgusting to even think about helping the blacks in their own community.
Mrs. Merriweather says she's upset with her black maid, Sophy, for being sulky and upset after the verdict of Tom's trial. Mrs. Merriweather said she told Sophy to cheer up, the black community had it coming, and if Sophy didn't cheer up, Mrs. Merriweather would fire her. A horrible way to treat someone who has just suffering a blow. Mrs. Merriweather also says she only keeps Sophy around because Sophy needs the money--which probably isn't true. Mrs. Merriweather seems like the type who likes to be waited on.
Lastly, Mrs. Merriweather begins to speak poorly of Atticus and his decision to defend Tom. She has the nerve to do this in Atticus's house, in front of Atticus's sister and daughter, while eating the food Attius's purchased with the money he was paid for defending Tom.
Because they are a church group that is supposed to get together and talk about church and what not. and all they do is sit around and talk about gossip, and all they do is discriminate.