Final answer:
Macduff calling Lady Macbeth a 'gentle lady' is ironic because her manipulative and ruthless character is the opposite of gentleness; this is seen in her planning of King Duncan's murder and scolding Macbeth for not properly framing the guards.
Step-by-step explanation:
When Macduff refers to Lady Macbeth as O gentle lady in line 80, it is considered ironic because Lady Macbeth is anything but gentle. The irony stems from her earlier actions and words, which reveal her as a manipulative and ambitious character, far from the traditional, gentlewomanly image. Macduff is unknowingly commenting on her appearance of fragility and nobility at that moment, unaware of her involvement in the treacherous acts that have unfolded. For example, Lady Macbeth is the one who crafts the plan to murder King Duncan, challenging Macbeth's manhood by saying, 'When you durst do it, then you were a man.' She also instructs Macbeth to 'look like the innocent flower, But be the serpent under 't,' demonstrating her willingness to deceive. Additionally, she takes control of the situation after the murder, telling Macbeth to wash the blood from his hands and scolding him for not leaving the daggers with the guards to frame them. Macduff's remark contrasts sharply with her proven character, thus the dramatic irony.