The most obvious sort of irony in this scene is sarcasm, used by Lennox to emphasize his distaste for Macbeth. This is the first scene in which Lennox appears after the ill-fated banquet scene in which Lennox observed Macbeth's behaviour upon seeing Banquo's ghost. Lennox and the others did not see the ghost, but he got a pretty good idea of Macbeth's role in both Banquo's murder and the murder of Duncan from this event.
Here are some of the ironic and sarcastic comments that he makes:
...The gracious Duncan
Was pitied of Macbeth. Marry he was dead.
And the right-valiant Banquo walk'd too late;
Whom you may say, if 't please you, Fleance killed,
For Fleance fled. Men must not walk too late.
Who cannot want the thought how monstrous
It was for Malcolm and Donalbain
To kill their gracious father? Damned fact!
How it did grieve Macbeth!
He goes on in ironic sarcasm to explain how "nobly" Macbeth has behaved and that he has "borne all things well." Shakespeare is using this ironic sarcasm to highlight Lennox's anger and disgust at the two-faced murderer, Macbeth.