How does the author's use of irony in "The Gift of the Magi" highlight the theme?

The theme of The Gift of the Magi is sacrifice.  Della and Jim each sacrifice something precious to them in order to give the other a special gift.  Della sells her hair and uses the money to buy Jim a chain for his beloved family heirloom watch.  Jim sells his watch to buy combs for Della's beautiful long hair.  The irony is that each person no longer possesses the item their gift was intended for.  Despite this sad irony, Della and Jim still appear happy.  They realize that their love for one another is the most important.  They experience each other's love through their sacrifices.  The irony highlights the sacrifice because the gifts cannot be used as intended.  If each had given a gift that could be used, the ending would have been only happy.  Instead, the ending is mixed with sadness.  Yet even in the sadness, there is joy and love.