What's ironic about Calpurnia saying she'll be warmer at her own house rather than at the Finches and what type of irony is it?
Calpurnia is black, and presumably she lives near the other black residents of Maycomb in "the Quarters". This part of town consists of rather neat, tidy, but very small cabins out past the city dump.
The Finch home, the reader can presume, is very nicely furnished, larger, more lavish and far more luxurious than all the cabins put together!
It is ironic that an upper-middle class home would not be able to be heated well enough to provide maximum comfort, whereas the small, snug cabins are just the perfect side to allow warmth and heat to spread throughout.
One would expect that the nicer homes in town would be the most comfortable, but in this case, it is the small cabins that are the warmest place to be on a cold evening.