In Macbeth, Act 3 scene 4, what's ironic about Macbeth's speech before the arrival of the ghost?

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:

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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.