The irony of the flashback
The ironic moment comes when Katie prays to God, saying that she will be happy with whatever fate they may have together, if only God will bless her with Johnny as her husband. The irony of course is dramatic irony, because she obviously didn't know what was going to happen with Johnny's future (or lack thereof). Then, there is the situational irony of Johnny himself being the worst part of their fate together—as if she had said, "I don't care who he is, just give him to me."
The irony of the ending
Another irony comes at the end of the book, in the form of Deus ex Machina, which simply refers to the outlandish redemption at the end of the story. In the story, this comes as a new marriage, showing the reader that the novel is not a tragedy, but a comedy. This reversal of fate (very often a marriage) is ironic in the same way any plot twist is ironic, especially since the book looks like it's about unlucky characters until suddenly, their lives become awesome.
The irony of parental roles
For whatever reason, many people believe that men are for earning money, and women are for raising a family. But for Katie, that's simply false. It's so false, she never even wonders for a moment about it, never hesitating to step in when it became clear that Johnny was not going to provide. The irony here is that what makes Katie a good mother is not her obedience to gender roles, but her willingness to do whatever it takes for her children. The true role of a parent is to sacrifice, which Johnny never really does. He never stops doing exactly what he wants, right until the end.
The irony of the death
The death of Johnny comes as a dark, backwards kind of blessing to the family, because life with a full-blown, unbridled alcoholic had been taking its tolls on the family the whole time. But in the end, his death is ironic in many ways. First, it comes from drinking, which is his very favorite thing to do for fun. So fun killed Johnny. But also, it was his aversion to responsibility, and although it's understandable why he might not want responsibility (sometimes responsibility is tough to stomach), it's certainly preferable to death. That's certainly an ironic fate, to drink yourself to death before thirty years of age. Ultimately though, the death is most ironic because it allows Katie to find a better man.
The sexual abuse incident
For Francie, the incident where a pedophile tried to rape her is already ironic in the implicit irony of her innocence. Having never thought about sex once in her life, she is suddenly exposed to the worst of the worst in terms of sexual deviancy. The real irony comes when Francie's mom shoots the man with her gun. Francie learns that her mom has a gun, first of all, and she learns that her mom is comfortable killing a man who would harm her. Katie turns out to be quite an edgy character.
