One of my favorite books!
In my perspective, the irony that is evident in this comes from society's view of happiness and success. Isn't it ironic that by societal standards, we may frown upon those who financially rely on Wal-Mart, yet our main character finds solace and happiness within the home she created based upon giving birth to her baby in the Wal-mart?
Another may be the irony of the triumph she gained of receiving home-ownership and success only after the tragedy of the tornado and loss of a dear friend.
These are just a couple starters for you...