Let us remember the three different types of irony that there are in literature before examining the end of this story. There is dramatic irony, which is when one of the characters and the audience knows something that other characters do not. There is verbal irony, when the opposite of what is meant is said, and finally there is situational irony, when the opposite of what we expect to happen occurs.
Thinking about the end of this excellent wartime story therefore, we can see that the way in which Fred Collins risks his life in the middle of battle with being blown apart all for one bucket of water automatically makes us assume that this costly prize will be enjoyed by the soldiers he is with. However, the precise opposite happens. The bucket full of water is dropped by two lieutenants who are joking around with it and the precious liquid falls to the ground. This is a classic example of situational irony.