Irony refers to the unexpected, and there is plenty of the unexpected in Edgar Allan Poe's classic tale "The Tell-Tale Heart," beginning with the fact that the narrator (who is also the killer) is only driven to homicide by his employer's eye, rather than the entire person. The ending is also ironic with the fact that the sound the narrator hears at the end of the story, and which drives him to tear up the floorboards and reveal his victim's corpse, is not audible to anyone else in the room.
Three different types of irony appear in literature: irony of situation, verbal irony and dramatic irony. The first type is the one that occurs in "The Tell-Tale Heart," as the second refers to people saying things that they do not mean, while the third refers to situations in which everyone (including the audience) knows what is going on except for one character.
The narrator is the servant of the person whom he kills, and it is some feature like a cataract in the man's eye that has driven the narrator to murder. When he goes in to look at the old man at night, when the eye is closed, he is unable to kill the man. It is the night when he makes a noise at the door, and the old man wakes up, revealing the eye, that the old man dies. It is the old man's scream that gets attention from the neighbors, but not before the narrator has time to chop up the body and hide it under the floorboards. Ironically, his own guilty conscience, combined with his insanity, compels him to tear up the floor because he claims that he hears his victim's heart, still beating.