In Chapter 14, as the band of travelers sneaks through the forest, Hawkeye tells Duncan about his experiences fighting against the French in the Seven Years' War. They are interrupted by a French soldier who asks them who they are; dramatic irony ensues as Duncan uses his French skills to deceive the soldier:
“Etes-vous officier du roi?”
“Sans doute, mon camarade; me prends-tu pour un provincial! Je suis capitaine de chasseurs (Heyward well knew that the other was of a regiment in the line); j’ai ici, avec moi, les filles du commandant de la fortification. Aha! tu en as entendu parler! je les ai fait prisonnières prés de l’autre fort, et je les conduis au général.”
This exchange roughly translates to:
"Are you an officer of the [French] king?"
"Without a doubt, my comrade; you thought I was one of the provincial [British] troops! I am a captain in the [French] cavalry (Heyward well knew that the other was of a regiment in the line); I have here, with me, the daughters of the commander of the fort. Aha! You've heard talk of them. I took them prisoner near the other fort, and I am taking them to the general [Montcalm].
Duncan may be a youthful and somewhat naive military leader, but he demonstrates quick thinking here. He technically does work for the king, just a different king than he leads the soldier to believe. He could have simply answered "oui [yes]," and given away as little information as possible, but instead he takes his answer as an opportunity to improvise an elaborate tale about who he is working for and what he is doing with these English-speaking people in the forest. He must have received a very strong education in the French language if he speaks well enough to convince this French soldier that he is a native speaker.
The exchange is shot through with dramatic irony because the reader and the rest of Duncan's crew know that he is making everything up. Up until the French soldier decides to let them pass, it seems quite possible that they are all about to be captured. The suspense not only gives Duncan a chance to prove himself in the face of danger, but it also drives the stakes of the entire plot higher. The forest is full of enemies, and the travelers need to be ready to contend with them at every turn. Chingachgook ends up sneaking off to kill the French soldier even after Duncan has convinced him to let them go. Cooper thus uses this encounter to depict stereotypical ways different cultures use for dealing with danger: Duncan, a white soldier, uses subterfuge, whereas Chingachgook, an American Indian, uses stealth and violence.
In Chapter 15, General Webb sends news that he is essentially abandoning Fort William Henry, which Munro has been holding against Montcalm's siege. Montcalm thus releases Hawkeye, who he had taken prisoner; when Duncan sees Hawkeye, there is a sense of situational irony in the scout's appearance:
The countenance of Hawkeye was haggard and careworn, and his air dejected, as though he felt the deepest degradation at having fallen into the power of his enemies. He was without his favorite weapon, and his arms were even bound behind him with thongs, made of the skin of a deer.
Hawkeye prides himself on his freedom from many colonial customs. A "man without a cross," he is not bound by Christian religion or European customs. Instead, he has spent most of his life in the wilderness with Chingachgook and Uncas. He is a skilled hunter and survivalist. It is ironic, then, that he now finds himself powerless in Montcalm's custody. He has been stripped of his weapon, which he calls "Kildeer." Symbolically, "his arms were even bound behind him with thongs, made of the skin of a deer." Part of the very animal Hawkeye is known for being able to shoot with uncanny accuracy is now being used by his enemy to tie him up.
Cooper does not humiliate Hawkeye in this passage to strip him of his status as a hero in the novel. Rather, the irony in this scene serves to demonstrate just how desperate circumstances have become at Fort William Henry. Montcalm has such an advantage that he has been able to turn the most unlikely person imaginable into his prisoner. Not only that, but Montcalm is so confident that Munro is no longer a threat that he is willing to release Hawkeye to him: Montcalm no longer feels any need for this highly valuable human bargaining chip.
In Chapter 21, Duncan stands at the ready to shoot a strange man Hawkeye is approaching. The scene is characterized by dramatic irony:
[...]Duncan turned his eyes just in time to perceive that a hundred dark forms were plunging, in a body, into the troubled little sheet. Grasping his rifle, his looks were again bent on the Indian near him. Instead of taking the alarm, the unconscious savage stretched forward his neck, as if he also watched the movements about the gloomy lake, with a sort of silly curiosity. In the meantime, the uplifted hand of Hawkeye was above him. But, without any apparent reason, it was withdrawn, and its owner indulged in another long, though still silent, fit of merriment.
Duncan does not understand why Hawkeye is laughing, nor why he seems unconcerned about the stranger. What Hawkeye, Uncas, and Chingachgook have all realized is that the strange man is David Gamut, the musician, and that the "dark forms" plunging into the waters are beavers he is trying to convert to Christianity. Hawkeye is laughing at the absurdity of David's proselytizing. It almost looks as though he is baptizing the beavers. Hawkeye also seems to be laughing because Duncan, who has yet to recognize David, is taking the encounter so seriously.
In this instance, dramatic irony provides comic relief in the midst of a suspenseful hunt for Cora and Alice. David and Duncan each appear woefully naive, which is all the funnier because they are each trying so hard at their respective jobs. David feels so intensely called to spread Christianity with his hymns that he can't distinguish between real potential Christians and beavers. Duncan, meanwhile, is so devoted to his role as a soldier that he is primed to shoot at someone so obviously harmless as David. This scene demonstrates that the forest is full of as many fools as fearsome enemies.