To me, this story is ironic because of the fact that Mr. Behrman dies from pneumonia. This is ironic because that is exactly what he is trying to save Johnsy from. She is dying of pneumonia and because she does not believe that she will be able to live once that last leaf falls.
So Behrman goes and paints a leaf on the wall. It convinces her to live, but painting it kills him because he went out in such bad weather to do it.
So my topic sentence would be something like "The Last Leaf is ironic because Mr. Behrman dies of pneumonia because he was trying to prevent that same thing from happening to Johnsy."
The previous editor gave you one sense of irony and I will present the other. Johnsy has willed herself to die and she keeps looking at the leaf on the vine that rests against the wall by her window. She states that she will die when the leaf is gone.
Day after day Johnsy watches the leaf. The longer the leaf hangs on the better she begins to feel until she is well. The irony is that the leaf was not real and she had willed her own self to live instead of allowing it to fall. Had she known that the leaf had already fallen, she would have died as well.
For me the story is ironic because Mr. Behrman was described as someone who was a failure when it came to his paintings given that he hasn't created a masterpiece in decades. Nobody expected him to create another masterpiece in his lifetime. But when he finally creates his masterpiece, it is something so powerful that it saved another person's life.