What is an example of irony in The Notebook?

In The Notebook, the irony lies in the fact that Duke, Noah's wealthy rival for Allie's love, is the one who ultimately helps bring Allie and Noah back together. Duke, though initially seen as a threat to Noah's relationship with Allie, turns out to be a catalyst for their reunion. It's due to Duke's actions and his possession of the unsent letters from Noah that Allie is prompted to reflect on her past love and the choices she made. This eventually leads her to reunite with Noah, the love of her life. The role reversal where Duke, the seemingly antagonistic figure, becomes a vehicle for the protagonists' happiness adds an ironic touch to the story.