There are a variety of ironies in this story. An irony is, simplistically, a contradiction. It is ironic that Delia makes her living by cleaning clothing, while her own clothes remain dirtied. It is ironic that Delia makes the money but that Bertha, Sykes' mistress, is the one to benefit from the money. It is ironic (and sad) that the town men complain about how Sykes treats Delia but fail to do anything about it. It is ironic that Delia is such a devout Christian, but that she must work on Sunday in order to survive. It is poetically ironic that Sykes purchases the snake to scare Delia, but it is the snake that kills him.
The ironies in this story help to underscore the message that not all conflicts and decisions are clear-cut, and that morality is not clear-cut. Is Delia wrong for not going to Sykes' aid? Is it wrong for her to work on Sundays? These are not clear choices. Delia is limited by her own life and by the society in which she lives. Hurston makes it clear that these ironies are pointing to greater injustices in the society as a whole.
Another irony would be that Delia is, in many ways, a very strong-willed, fiery woman who wants to be able to stand up to her abusive husband; however, she cannot do so due to societal rules during this time period. She knew she would be killed if she tried to leave her husband and/or truly "stand up" to him. Her "power," then, is that she doesn't assist her husband when he is bitten by the snake...she lets him die. In this way, she finally has power of her husband.