The Poisonwood Bible abounds in irony. What is the irony in Kinsolver's novel and how does it both reveal the theme and act as a purpose?

The ultimate irony in The Poisonwood Bible revolves around the death of the youngest daughter Ruth May.  The village in which the Price family lives is plagued by ants and two of the daughters run to safety.  Adah, who is handicapped, and Ruth May are left behind.  Orleanna reasons that when in trouble a mother needs to care for her children from the bottom up, so she saves Ruth May and leaves Adah behind.  Adah is then saved by a man in the village.  Later, someone plants poisonous snakes outside the Prices' home, and one bites and kills Ruth May, the daughter whom Orleanna tried to protect.  This is significant because it is at this point that Orleanna decides to take control of her family and leave the Congo.  Nathan Price has not listened to Orleanna's prior pleas to return home, and Orleanna has accepted her husband's desire to continue his mission.  Now, the death of their daughter forces Orleanna to take action.