The irony of the defeat of Arawn, the evil sorcerer
Arawn assures the Sons of Don (a fierce army) that no powerful enemy can defeat him. When the war breaks, the Sons of Don confidently start the battle, believing Arawn will emerge as the winner. Ironically, Prince Gwydion strategically planned for the battle and identified all the weaknesses of Arawn and King Math. For instance, Gwydion discovers that Arawn's soldiers cannot fight on the sea. Therefore, Gwydion's troops attack from the sea and successfully destroy Arawn, an outcome that was never expected.
The Irony of Taran’s decision to remain in Prydain's kingdom
Towards the end of the novel, Taran is given an excellent opportunity in his lifetime to move to Summer Country, where he will become immortal. Ironically, Taran refuses and prefers to be mortal because he cannot stay away from his wife. The reader keenly observes that throughout the novel, most characters want to go to the Summer Country because that guarantees them immortality. Consequently, the decision of Taran to choose mortality is paradoxical.
The irony of Eilonwy
There is a situational irony when Eilonwy agrees to give away her magical powers to remain with Taran. Eilonwy has had magical power throughout her life and has managed to remain victorious in anything she does. The decision to give away her magical power to get married to Taran is ironic.