What do you understand by dramatic irony ? What use has Shakespeare made of it in Macbeth?

Dramatic irony in literature occurs when the audience
understands or is aware of something but the character(s) are not clued
in.

In Macbeth, the play begins,
continues, and ends with dramatic irony. Consider the witches' prophecies in Act 1,
Scene 3:

“All
hail, Macbeth! that shalt be king
hereafter.”

Macbeth will be
king, all right, but surely his reign is not the glorious experience he had expected.
Thinking about audiences in Shakespeare's day, they would have already been wary of the
witches as soon as the Weird Sisters appeared. They knew that nothing good could come
from evil. The murder of Duncan in order for Macbeth to ascend the throne is the evil
that results from Macbeth's lust for power and his belief that he is fulfilling his
destiny.

There are hints throughout Act 2 that things are
not going as planned, but the audience sees it more clearly than do the characters. By
Act 3, Lady Macbeth is starting to catch on. She tells her treacherous
husband:

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Nought’s had, all’s spent. Where our desire is
got without content: ’Tis safer to be that which we destroy than, by destruction, dwell
in doubtful joy.

The joy is
"doubtful" because the spoils of the kingdom are spoiled; the throne was won through
bloody murder, not divine right, not by just
means.

Dramatic irony is also present in the references to
water and guilt. While it would be hundreds of years before Carl Jung, the renowned
psychoanalyst postulated his theories of archetypes, Shakespeare seemed to intuit those
theories. Water, according to Jung, is symbolic the collective unconscious, of birth and
death, and of purification and redemption.

In Act 2, after
the murder of Duncan, Macbeth exclaims,

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Will all great Neptune’s ocean wash this blood
clean from my hand?

Neptune,
of course, was the god who ruled the oceans, but interestingly, he is associated more in
Roman mythology with fresh water than sea water. This "fresh water" is symbolic of
Macbeth's hopes to be "purified" and "redeemed" but the audience is aware that there
will be no forgiveness for the corrupt king.

Lady Macbeth,
too, uses the archetype of water for her own complicity in the evil deeds. After
Macbeth's Neptune plea, his wife replies in a dismissive
way,

A little
water clears us of this
deed.

Again, the audience is
aware that such a heinous crime will not so easily be dismissed, and they, of course,
are correct. Later, Lady Macbeth's "collective unconscious," her guilt, consumes her. In
her infamous sleepwalking scene, we see her desperately trying to "purify" herself of
her crimes:

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Out, damned stop! out I say!
...

What, will these hands ne’er be
clean?...

Here's the smell of blood still. All
the

perfumes of Arabia will not
sweeten

this little hand. O,
O!

From beginning to end, the
audience knows that the Macbeths will achieve their bloody ambition, but they will not
be happy, they will not enjoy their ill-gotten throne, and that they will never be free
of the guilt spawned from their despicable crimes.