I believe the correct answer is: Because Ismene was actually afraid to help Antigone.
As a figure of speech, irony primary represent the use of one's language that normally signifies the opposite. But, it can also represent the state of affairs or an event that seems deliberately contrary to our expectations.
Therefore, in this case of Sophocle’s play “Antigone”, Ismene denies the help to Antigone to bury their brother, Polyneces, because she is afraid to help. But, never the less, Ismene is imprisoned by Creon, alongside Antigone. This is ironic because the audience/readers would not expect for Ismene to be imprisoned as she is abiding the law out of fear.
