In Macbeth, Banquo says: "I dreamt last night of the three weird sisters: to you they have showed some truth." How is this an example of irony?

I think if you are looking for irony in this direct quote
of Banquo's you need to explore what Banquo is really saying to Macbeth in this passage.
Let us remember that Act II scene 1 opens with Banquo, who, like Macbeth, is unable to
sleep because of his meeting with the witches. Note what he
says:

A heavy
summons lies like lead upon me,

And yet I would not sleep:
merciful Powers!

Restrain in me the cursed thoughts that
nature

Gives way to in
repose!

This is a fascinating
speech because it shows that Banquo, just like Macbeth, is haunted by the promises of
the witches and what they have prophesied. The key difference, of course, is that
Macbeth chooses to act on those prophecies to make them real, whereas Banquo does not.
However, Banquo is wondering in this scene how Macbeth is responding and reacting to
these prophecies, especially because the first prophecy they gave him has been confirmed
with Macbeth's title of the Thane of Cawdor. The irony in the quote you have highlighted
therefore is that Banquo is trying to work out what Macbeth is thinking, planning and
plotting. If Banquo is so plagued by what has happened, then how will Macbeth be
responding, who was promised the crown?