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?
Banquo's statement comes just before Macbeth murders Duncan; Macbeth's visit to Banquo's chamber is to find out if he can trust him after the murder for he asks him
If you shall cleave to my consent, when 'tis,
It shall make honor for you.
Banquo's response is
So I lose none
In seeking to augment it, but still keep
My bosom franchised and allegiance clear,
I shall be counselled.
Macbeth has his answer; he cannot trust Banquo.
However, the irony here is situational, for we know that Macbeth plans on making the prophecy come "true" by murdering Duncan, something that Banquo does not even suspect. Therefore, Banquo's statement is quite irony considering the timing of his statement.