eNotes defines dramatic irony explaining that it is...
...when words and actions possess a significance that the listener or audience understands, but the speaker or character does not.
In Shakespeare's Macbeth, when Duncan and Banquo speak in Act One, scene four, they are actually talking of things that have a much deeper significance than either could imagine at that time.
There is dramatic irony because Macbeth has been receiving strong praise for being so worthy on the battlefield, while Banquo seems to be considered of lesser importance to the King. (This might be explained in that Duncan and Macbeth are cousins, while Banquo is not related, and therefore does not share the same relationship with the King.) The truth is that Banquo turns out to be the better, greater man. It is ironic that whereas the more highly praised Macbeth says all the right things, it is Banquo, the "secondary" hero, who means what he says with all of his heart. Macbeth proves soon to be one that cannot be trusted, but Banquo (as should Macbeth) would willingly lay down his life for King and country.
Banquo and Duncan use a "plant" metaphor as the King praises Macbeth and then Banquo:
I have begun to plant thee, and will labor
To make thee full of growing. Noble Banquo,
That hast no less deserved, nor must be known
No less to have done so... (I.iv.28-31)
Banquo responds to this pledge by continuing with the same kind of metaphor:
There if I grow,
The harvest is your own. (33-34)
The irony is that they expect that the kingdom will prosper with Banquo's continued help, and Banquo will prosper in serving his King. Neither realizes that he will be murdered by Macbeth or his "minions" and never realize the "harvest" they both speak of.
In Act One, scene six, as Duncan and Banquo enter the courtyard to the Macbeths' castle, they comment on how lovely a place it is. Ironically, Banquo notes that there are martlets nesting there, and his experience has been that they nest only where the air is "delicate." One gets the sense that the birds "cradle" their young in the safest of places. Nothing could be further from the truth. This will turn out to be a place of death for both men.