The irony arises from the fact that the very ambition and ruthlessness that drove her in the beginning ultimately become her undoing. The guilt of murdering Duncan and framing others for the crime starts to eat away at her conscience. She becomes haunted by visions and begins to experience sleepwalking episodes. Her famous sleepwalking scene in Act V, where she tries to wash away the imaginary blood from her hands, is a powerful symbol of her internal torment and the unravelling of her mental state.
The irony is heightened by the contrast between her earlier confidence and her eventual descent into madness. Her mental breakdown reveals the fragility of her strength, as she becomes a mere shadow of her former self. This transformation from a resolute and ambitious woman to a guilt-ridden and psychologically broken individual serves as a poignant reminder of the consequences of unchecked ambition and the destructive power of guilt.
Moreover, the irony extends to the fact that Lady Macbeth becomes the victim of her own scheming. Her desire to be free from guilt leads her to seek solace in sleepwalking, yet it is during these episodes that she exposes her guilt and incriminates herself and her husband. The very actions she takes to escape her mental anguish end up further tormenting her and sealing her fate.
Overall, the irony in Lady Macbeth's mental state lies in the contrast between her initial strength and determination, and her eventual transformation into a guilt-ridden and shattered individual. She becomes a cautionary tale about the perils of ambition and the destructive consequences of ignoring one's conscience.