This novel is filled with different types and examples of irony. The most obvious examples are found within the ending of the novel. It is great situational irony that Ethan intends to commit suicide with Mattie, but ends up merely crippling both of them. This seems worse than death, especially considering the ironic change in Mattie's behavior. Where she once was a light and caring person, she has aged into a bitter and cantakerous older woman who is miserable her in wrecked body and wrecked life.
It is ironic that the once very feeble and sickly Zeena is now the strong and capable woman at the end of the novel. It would seem that she is very able-bodied when she needs to be. Once Ethan and Mattie need her to care for them, she rises to the occasion. It is a revisiting of the plot element that brought her to the Fromes in the first place. She is caring for an invalid again.
It is ironic that Ethan was unwilling to leave Zeena and run away with Mattie, but his choice is now a double punishment because he is still with Zeena, but he also has to daily face the invalid Mattie and a) remember what she used to be to him, and b) put up with her negatively changed behavior.
Other ironic occurances:
1.Mattie only reveals that she loves Ethan when he driving her out of town to catch a train. It seems to suggest a manipulative nature in Mattie, but it ends up destroying them both.
2. Ethan only asks Zeena to marry him because he can't bear to face a winter alone. If his mother had died in spring or summer, he would likely have returned to school and left Starkfield for good.
The list could obviously go on, but these are a few big and small ideas to get you started!