Grendel is the 1971 novel by American writer John Gardner. It is a retelling of the famous epic poem Beowulf from the perspective of the antagonist, Grendel. This novel presents the titular character as an antihero and explores the nature of good and evil in an existential manner.

There is dramatic irony in chapter 9. Dramatic irony occurs when the reader has more information than some of the characters, which changes the meaning of interactions. Grendel has a habit of attacking and killing priests and those associated with religion. The reader also knows that Grendel intends to kill the priest Ork. When Grendel asks Ork about the nature of religion, the meaning is different for Ork and the reader as the reader expects an imminent attack from Grendel.

There is another piece of irony in chapter 9 when Grendel admits:

Something is coming, strange as spring.
I am afraid.
Standing on an open hill, I imagine muffled footsteps overhead.

The reader understands this “something” to be Beowulf, the hero who will eventually kill Grendel. At this point in Grendel’s life, he does not know about Beowulf. The reader picks up on this ironic foreshadowing while Grendel is left both confused and afraid. In Beowulf, Grendel is presented as inherently evil, so this confusion and fear work to add depth to Grendel’s character, and the audience views him in a more forgiving light than in the original Old English poem, which is in itself ironic.

Verbal irony occurs when an utterance means the opposite of what is intended. In chapter nine of Grendel, Grendel thinks about how religion has changed in his time. He remembers an old peasant saying that, in former days, the priests used to sacrifice virgins, but don't any longer. "Religion is sick," the peasant says. Grendel silently agrees. The peasant means that religion is too diseased, too weak, to nobly kill virgins as it once did. Of course, we as a modern audience interpret "religion is sick" to mean it is diseased because it once killed virgins, not that it stopped doing so. Our interpretation is the opposite of how the peasant expected his words to be taken.

Dramatic irony occurs a little later in the chapter. Dramatic irony occurs when an audience knows what a character or characters in a work of literature do not. We know what the old priest Ork doesn't: Grendel has mentioned to us he plans to kill him, deciding that he will "paint ... with the old man's steaming blood." Grendel asks him questions about religion merely to play with him and probably to ignite his own anger, so he can more easily murder Ork. This leaves the reader on edge, expecting Grendel to strike at any moment. However, the genuine passion of the priest's response adds another layer of irony, or reversal of expectation. Ironically, Grendel, who has a history of acting aggressively towards religion and priests, does not strike. 

In chapter nine of Gardner's Grendel, irony is present in the interactions between Grendel and the priests:

"Great spirit," the chief of the priests wails, "ghostly Destroyer, defend the people of Scyld and kill their enemy, the terrible world-rim-walker [Grendel]!"

The priest prays for Grendel's destruction while Grendel listens and waits for someone to come kill him, which is ironic.  But the irony increases when Grendel poses as the Destroyer himself. 

At midnight, with Grendel sitting in the ring of the gods, Ork comes and asks who is there.  Grendel replies:

"It is I," I say.  "The Destroyer."

Grendel, the nihilist and atheist, poses as the god the priests pray to for Grendel's destruction.  That is ironic.

The irony is heightened still further when Ork sees his visitation or vision as confirmation that he is correct in his theories and beliefs.  Grendel, the being who detests the pattern-making of humans, laughingly contributes to the confirmation of the patterns envisioned by Ork.