Irony as a literary device is when a situation is is unanticipated or contrary to what is expected or is the reality. When the unexpected occurs it can be a humorous or tragic situation. So it is used in both Comedy and Tragedy.
An example is in the case of the titular characters of Shakespeare's play Romeo and Juliet where the young lovers thinking the other is dead end up killing themselves, all the while the audience aware of the situation are left to feel helpless. This is Dramatic irony.