In Poe's, "The Cask of Amontillado", what does Montresor mean by, "But I must first render you all the little attentions in my power", right before...

"But I must first render you all the little attentions in my power", is something a host might say to their guest. In a normal situation, the host would want to give the guest anything they might need: blankets, towels, toiletries, etc. But what "little attentions" could you give a man who you were about to sentence to a prolonged death? A faster death? No, in this case, Montresor's words are said sarcastically because he does not actually have anything to give Fortunato. In its own way, sarcasm is a form of irony, as you say one thing but mean something completely different. In this case, Montresor is promising to be a good host to Fortunato, but in fact he is getting the bricks and mortar ready to wall Fortunato into the niche permanently, which is decidedly not something a good host would do.

Although sarcasm is used in everyday speech, I provided a couple of links explaining its usage in literature.