What, in Macbeth, is the symbolic meaning of Lady Macbeth's unconsciousness and why is it ironic?

I believe that you are referring to Lady Macbeth's somnambulism and her actions during this state.
The lady's mental deterioration is symbolic of the destructive power of evil. Evil seeks to destroy and overturn the natural order. Its power is vested in its utter contempt for good and a depraved and ruthless desire for authority driven by greed, arrogance and self-aggrandizement.
Those who ruthlessly seek power turn to evil means to get what they want. The forces of evil which they seek out and believe in, provide them a shortcut to fulfill their pernicious desires. So it was with both Lady Macbeth and her husband. Note, for example, her plea to the dark forces when she decides to murder king Duncan, in Act 1, scene 5:

Come, you spiritsThat tend on mortal thoughts, unsex me here,And fill me from the crown to the toe top-fullOf direst cruelty! make thick my blood;Stop up the access and passage to remorse,That no compunctious visitings of natureShake my fell purpose, nor keep peace betweenThe effect and it! Come to my woman's breasts,And take my milk for gall, you murdering ministers,Wherever in your sightless substancesYou wait on nature's mischief! Come, thick night,And pall thee in the dunnest smoke of hell,That my keen knife see not the wound it makes,Nor heaven peep through the blanket of the dark,To cry 'Hold, hold!'

Her dramatic cry is truly terrifying. Once she has made up her mind, she becomes unstoppable and even goes as far as mocking her husband for being so feeble and cowardly. She uses powerful and persuasive language to convince her unsure husband that they should commit their heinous crime. She is so forceful that he is swayed to state that she should 'bring forth men children only' since they would bear her courage and purpose.
It is, therefore, ironic that Lady Macbeth was the first to falter when she saw the depth of destruction she had unleashed. In persuading her husband, she had opened a Pandora's box of evil. Macbeth became unrelentingly ruthless. He turned into a tyrannical monster intent on destroying all those who opposed his will, no matter what. He had his closest friend and confidante, Banquo, killed because he deemed him a threat and had Macduff's entire family assassinated for the same reason. Also, Macbeth had become so intent on waging a war with his own people that he had very little time for her.
Lady Macbeth became increasingly paranoid. She was driven by guilt and would attempt to remove unseen stains from her hands. She would constantly have a light with her and mutter about the dark deed she and her husband had committed. She was overwhelmed by remorse and the trauma of what she had done eventually drove her to madness and then suicide.
What she probably did not realize was that evil, being a destructive force, also destroys those who seek and then embrace it. She and her husband became tools to spread mayhem and both of them became, ironically, victims of their own malice.