The irony in Chapter Nine of Book the Second lies in the way that the sunlight actually foreshadows the act of murder that has been committed during the night. Let us consider the following paragraph from this excellent chapter so that we can discuss how the irony in it functions:
Lighter and lighter, until at last the sun touched the tops of the still trees, and poured its radiance over the hill. In the glow, the water of the chateau fountain seemed to turn to blood, and the stone faces crimsoned. The carol of the birds was loud and high, and, on the weather-beaten sill of the great window of the bedchamber of the Monsieur the Marquis, one little bird sang its sweetest song with all its might. At this, the nearest stone face seemed to stare amazed, and, with opened mouth and dropped under-jaw, looked awe-stricken.
This quote of course immediately precedes the discovery of the corpse of the Marquis, and thus the descriptive detail of the water in the fountain turning to blood has ironic significance. In addition, there is irony in the way that the little bird sang "its sweetest song with all its might" at the window of the bedchamber of the Marquis, only to have this excellent song fall on deaf, or dead, ears. The gargoyle faces themselves ironically appear to be impacted by the act of murder, as they are described as being "awe-stricken."