The Thane of Ross's description of the current state of affairs in Scotland constitutes situational irony. Situational irony is created when a situation or event is different from, or even opposite of, what we would expect. He says,

But cruel are the times when we are traitors
And do not know ourselves; when we hold rumor
From what we fear, yet know not what we fear . . . (4.2.22–24)

In other words, people are accused of being traitors but have no idea why or what they are accused of doing. Further, people fear the rumors they hear, but they do not know what they should actually be afraid of. One would expect an accused traitor to know what they've done or a fearful person to know what they fear, and so this situation—wherein they do not know—constitutes an irony. This situational irony emphasizes just how corrupt Macbeth's leadership is and how terribly the country has deteriorated under Macbeth's reign.

Ross continues, saying that the citizens of Scotland "float upon a wild and violent sea" (4.2.25). Now, they are not literally floating in the ocean, so we can identify this figure as a metaphor. Macbeth's paranoia and ambition have created such a terrifying reality that Ross compares the corruption of the state to a dangerous, menacing sea that threatens to drown those within it. Again, this emphasizes how bad things are under Macbeth's leadership.

Shakespeare uses an extended bird metaphor throughout Act IV, scene two in Macbeth. The comparison of man to bird begins immediately as the scene opens and Lady Macduff despairs that her husband's "flight was madness" (IV.2.4). 

She goes on to explain to Ross how parents should defend their children, using a wren as a metaphor and example:

From whence himself does fly? He loves us not;
He wants the natural touch for the poor wren,
The most diminuitive of birds, will fight,
Her young ones in her nest, against the owl. (IV.2.11-14)

In Lady Macduff's mind, her husband has acted against nature.  She feels his abandonment keenly.  Her use of the wren metaphor, however, also suggests that she identifies herself to the wren, "the most diminuitive birds," suggesting that she will not run away as her husband has. 

Shakespeare's use of the extended metaphor continues as the son enters the conversation.  When his mother asks him how he will survive without his father, he responds "As birds do, Mother" (IV.2.37).  This comment opens their discussion into more word-play about birds and survival.