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

Irony is generally defined as being when something happens that is unexpected. In "The Fall of the House of Usher," Edgar Allan Poe uses a technique called ironic reversal, which means that he shows an impossibility in one part of a text, then makes it a possibility in another part of the text, making the reader think of the original impossible idea and how it was not expected to ever become true.

Specifically, the narrator is coming up to the House of Usher and sees it reflected - and consequently upside-down - in the "black and lurid tarn," which is a contaminated mountain lake. 

Then, at the end of the story, the narrator runs out of the house and watches as it literally falls into the tarn and is submerged by it.