The concern for fortunato is ironic because Montresor intends to do harm to fortunato. Have in mind that Montressor needs to make sure that his plan goes off without a problem. If Montressor were to act indifferent to Fortunato's health, Fortunato may suspect that Montressor was there not to do good to fortunato. Remember that Irony is a state of affairs, a comment, or an event that is contrary to what you expect.
