The irony of Joe and Janie's relationship at this point in the novel connected to the central theme of finding one's voice. Joe, who has always promoted himself as a "big voice" in Eatonville, has basically suppressed Janie's voice throughout their entire 20 year relationship. And now, on Joe's deathbed, the full weight of this implication is realized.
As Janie says:
"Dat's just whut Ah wants to say, Jody. You wouldn't listen. You done lived wid me for twenty years and you don't half know me atall. And you could have but you was so busy workhippin' de work of yo' own hands, and cuffin' folks around in their minds till you didn't see uh whole hep us things yuh could have."
"Leave heah, Janie. Don't come heah--"
"Ah knowed you wasn't goingtuh lissen tuh me. You changes everything but nothin' don't change you--not even death [...] Mah own mind had tuh be squeezed and crowed out tuh make room for yours in me."
In this scene, Janie finally raises her own voice and speaks her own mind, re-connecting with her soul and her sense of identity. Unfortunately for Joe, it takes death to silence him long enough to hear what Janie has wanted to say for all these years.
This is situational irony.