In the Tell-Tale Heart, the situational irony occurs when the old man (who is very rich), lives in torment fearing that robbers would go and steal his gold and many possessions, so he keeps his windows and everything down to protect himself.
Yet, the narrator has been growing obsessed with the old man's eye which was cloudy and the narrator thought it was evil. The irony comes when this same narrator kills the man because of this obsession. It is ironic because many critics that have analyzed the story think that the narrator was either living with the old man, or was familiar to him by the way that the old man reacted when the narrator attacked him.
Hence, it is ironic that, as he tried to protect himself from the outside world, evil was growing and getting ready to attack him from the inside.
It is ironic that he feared robbers because it was someone in his home that he should have feared more. Poe makes us understand that the voluble murderer has been tortured by the nightmarish terrors he attributes to his victim:
‘‘He was sitting up in bed listening;—just as I havedone, night after night, harkening to the death watches in the wall’’;
further, the narrator interprets the old man’s groan in terms of his own persistent anguish:
‘‘Many a night, just at midnight, when all the world slept, it has welled up from my own bosom, deepening, with its dreadful echo, the terrors that distracted me.’’
Thus, Poe, in allowing his narrator to disburden himself of his tale, skillfully contrives to show also that he lives in a haunted and eerie world of his own demented making.