What's ironic in "The Fall of the House of Usher"?

Critic G. R. Thomson is widely quoted for having called "Usher" a "masterpiece of dramatic irony." Dramatic irony means that the audience is more aware of what will happen in a story than some of the characters themselves. For example, in Macbeth, we as the audience know that Macbeth will murder Duncan because we witness the scene where he agrees to do so, but Duncan himself arrives at the castle trusting Macbeth and blithely unaware of his fate. In "Usher," the irony is not as clear-cut as in Shakespeare, bleeding as it does into foreshadowing, but it still exists.

Roderick lives in a state of terror, in dread of the future, to such an extreme degree that he is physically and mentally debilitated. His nerves are endlessly on edge. It's not hard for the reader to understand that a person living in such an unstable condition is, ironically, almost certain to bring on himself through his stress the very state of abandoning "life and reason" that has led to his terror. Ironically, if Roderick had feared less, he might have had less to fear.

The narrator clearly describes Roderick's state:

To an anomalous species of terror I found him a bounden slave. “I shall perish,” said he, “I must perish in this deplorable folly. Thus, thus, and not otherwise, shall I be lost. I dread the events of the future, not in themselves, but in their results. I shudder at the thought of any, even the most trivial, incident, which may operate upon this intolerable agitation of soul. I have, indeed, no abhorrence of danger, except in its absolute effect—in terror. In this unnerved, in this pitiable, condition I feel that the period will sooner or later arrive when I must abandon life and reason together, in some struggle with the grim phantasm, Fear.” 

Of course, at the end Roderick does come to a bad end, dying from witnessing his sister's bloody escape from the vault. As the narrator states, his sister:

with a low moaning cry, fell heavily inward upon the person of her brother, and in her violent and now final death-agonies, bore him to the floor a corpse, and a victim to the terrors he had anticipated.