The narrator of "The Cask of Amontillado by Edgar Allan Poe is Montresor, a man who wants nothing more than to exact his revenge on a man named Fortunato. The plan is elaborate and takes place during Carnival; however, it will only work if he can manage to get rid of his servants. Because he knows them well and has chosen his time wisely (during the Carnival season), he thinks he can make it happen.
I had told them that I should not return until the morning, and had given them explicit orders not to stir from the house. These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned.
He says he ordered them not, under any circumstance, to leave his house, even though he was going to be gone all night and into the next morning. In a bit of irony, Montresor knows that this is just the thing his servants need to hear in order to ensure that they will promptly leave as soon as their employer leaves.
This is an ironic statement, then, which is a contrast between what he said and what he expected to happen. And, of course, they did just as he anticipated they would, leaving him free to pursue his plan to exact revenge on Fortunato.