The conclusion of Ethan Frome is deeply ironic for a number of reasons, and this irony amplifies the novel's tragedy.
At multiple points throughout the novel, Ethan wishes for Mattie to remain at the farm forever. In Chapter 2, he even fantasizes about the two of them dying and being buried next to each other in the family graveyard:
"I guess we'll never let you go, Matt," he whispered, as though even the dead, lovers once, must conspire with him to keep her; and brushing by the graves, he thought: "We'll always go on living here together, and some day she'll lie there beside me."
By the end of the novel, Ethan has gotten what he wanted. Mattie will live at the farm for the rest of her life, and she will be buried in the family plot. But this situation brings Ethan no happiness—Mattie is permanently crippled as a result of the accident, and she and Ethan are never able to live together as man and wife. When Mattie and Ethan die, it will not come at the end of a long and happy life, since their current situation now resembles a kind of living death.
This conclusion is also ironic because Ethan initially falls in love with Mattie because she is so different from Zeena. After the accident, however, the women have become nearly identical in terms of appearance and personality. In his attempt to escape his marriage to Zeena, Ethan has actually made his situation worse than it was before.