Final answer:
Lady Macbeth's ironic reaction is shown through her fainting during the chaos she helped create, juxtaposed with her earlier determination and resolve to be unemotional and 'unsexed.' Macduff's words echo this when he says reporting Duncan's murder might be too much for a woman to bear.
Step-by-step explanation:
The irony in Lady Macbeth's reaction to the murder news in front of the noblemen is that she faints or appears to be overwhelmed by the chaos, thus diverting attention from Macbeth's odd behavior after Duncan's murder.
This is ironic because earlier in Macbeth, she was the one who planned the murder and chastised Macbeth for his lack of resolve, showing a dichotomy in her character's public and private personas.
Moreover, the words that Lady Macbeth used earlier, beckoning spirits to 'unsex' her and strip away her femininity so she could commit cruel acts without remorse, are ironically echoed by Macduff's suggestion that the news of Duncan's murder is too brutal for a woman's ear — 'The repetition, in a woman's ear, Would murder as it fell.' — which contrasts Lady Macbeth's earlier desire to be more 'male' and unemotional about such cruelty.