Prior to the banquet, Macbeth has arranged for Banquo and his son, Fleance, to be murdered. This act of betrayal reveals Macbeth's manipulative and power-hungry nature. When Macbeth raises his cup and proposes a toast to Banquo, the audience is aware that Banquo is no longer present due to Macbeth's own deceit.
The irony lies in the fact that Macbeth is publicly honoring and praising Banquo while simultaneously being the mastermind behind his impending demise. Macbeth's toast becomes a façade to conceal his true intentions, adding a layer of deceit and hypocrisy to the scene.
Furthermore, the ghost of Banquo appears during the banquet, visible only to Macbeth. This supernatural element further emphasizes the irony of the situation as Macbeth's guilt and paranoia manifest before him while he attempts to maintain an outward appearance of normalcy.
The toast to Banquo serves as a commentary on Macbeth's internal struggles and the dissonance between his actions and public persona. It underscores the tragic consequences of his choices and highlights the ironic nature of his downfall.