Dramatic irony is dependent on the audience being aware of more information or the outcome moreso than the characters are. In this situation it is ironic that they are discussing the turn of events and that Macbeth is being promoted. Banquo appears to be a bit jealous in some respects because he, too, engaged in much the same sort of heroic endeavors on the field of battle. The discussion about the weird sisters brings home the point that the prophecy is beginning to unfold.
