Final answer:
The statement is ironic because the house is not what it appears to be and the upcoming murder is unknown to Duncan.
Step-by-step explanation:
Duncan's statement that the house looks pleasant is ironic because it contrasts with the actual state of the house, which is described as a decrepit castle. This creates a sense of dramatic irony, as the audience knows something that Duncan does not - that the house is not what it appears to be. The irony adds tension and foreshadows the upcoming murder that Duncan is unaware of.