What is ironic about the inscription on the pedestal and statue?

duncan's speech on his arrival at inverness is heavy with dramatic irony: not only is the "seat" (the surroundings) of the castle "pleasant," but even the air is sweeter than that to which the king is accustomed. the presence of the martlet (a summer bird) serves to heighten the irony. as far as the king is concerned, the castle, from the outside at least, appears to be a paradise. contrast this picture of delight with the imagery of hell that forms the substance of the porter scene (act ii, scene iii).