The link below has a great answer that might also help when answering this question. It deals with Lady Macbeth's state of mind during her sleepwalking spells.
Also, remember that Irony is when something unexpected happens when someone is actually trying to stop it from happening. So as you read the scenes about Lady Macbeth, consider what she is trying to prevent, but it actually happens. You can also look at the opposite of that too. What is she trying to create or hold on to, but it slips away? And, usually things that are ironic happen because of something completely out of the control of the character. Sometimes nature gets in the way of what a character wants to have happen; sometimes the actions of the character himself/herself actually gets in the way of what they want to achieve.