Let us remind ourselves of the context of this scene. Macbeth has just found out from the Murderers that Banquo has been killed by his orders. He then rejoins his party, trying to play the part of the host and benificent King. Note what he says to the assembled masses:
Here had we now our country's honour roof'd,
Were the grac'd person of our Banquo present;
Who may I rather challenge for unkindness,
Than pity for mischance!
Macbeth is saying that if Banquo were present he would have the best and most noble members of his country all under the same roof. However, it is clear that this is verbal ironly in a number of levels. Having just arranged Banquo's assassination, on the one hand it is unlikely that Macbeth will believe this. On the other hand, the irony could actually run deeper, as this speech may operate at some level to express the way in which Macbeth recognises that Banquo is a much better man than he is, and there is truth therefore in what he says. Either way, the irony of his speech lies in wishing for the presence of Banquo overtly whilst secretly he knows that Banquo has been murdered.