From my perspective that irony in this piece is that Dulce Rosa swore vengeance on the man who killed her father and rapped her. To our surprise she falls in love with the man that killed her father and decides to marry him. Then, she kills herself so she does not see or hear the ghost of her father anymore who she believes is haunting her. Tadeo must pay for his guilt and mistakes and live alone until he dies of old age, alone, only having love for Dulce Rosa.
(So to sum it up, the irony is she falls in love with her father's murderer. And instead of killing his murderer, she kills herself.)
(So to sum it up, the irony is she falls in love with her father's murderer. And instead of killing his murderer, she kills herself.)