What is ironic about lady Macbeth's statement What is done is done?

Lady Macbeth's statement "What is done is done" is ironic because she says it to try to reassure herself and her husband after Duncan's murder, when in reality, the consequences of their actions continue to haunt them throughout the play. The statement reflects Lady Macbeth's attempts to suppress her guilt and anxiety by dismissing the gravity of their crimes, which ultimately leads to their downfall.