How does Poe use irony to convey a theme of flattery?

Montresor's treatment of Fortunato in Poe's The Cask of Amontillado is laced with irony. In large part, the irony develops the theme that flattery can be weaponized to hurt the truly vain. Montresor uses his knowledge of Fortunato's excessive pride and is able to manipulate him through flattery in order to enact his dreadful revenge.
When Montresor first meets Fortunato, he says, "My dear Fortunato, you are luckily met." Because of his inflated sense of self, Fortunato assumes Montresor is happy to see him because of his expertise in tasting and judging wine and brandy. In reality, Montresor felt their meeting is "lucky" because the carnival presents the perfect opportunity for murder. Ironically, Fortunato believes the reference was intended as a compliment, when in reality Montresor was planning his death.
This weaponization of flattery can also be seen in Montresor's thoughts, "It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation." Fortunato is pleased with himself and is so proud to be in Montresor's good graces that he is blinded to the fact that Montresor's thoughts are elsewhere. It is ironic that Fortunato thinks he can verify the Amontillado's authenticity when he is unable to properly identify the lack of authenticity in Montresor's behavior towards him.
Fortunato's own drunken state, coupled with his weakness for flattery, leave him unable to judge the wisdom of continuing down the winding paths further and further into the catacombs. Montresor brilliantly convinces him to continue further by suggesting that Fortunato seems too sick to continue on the journey. Again, Fortunato's own sense of pride would not allow him to admit the cough was getting worse and that his health could interfere in his tasting of the Amontillado. By telling him he shouldn't do it, Montresor was ironically goading him closer to his death.
The reader never learns exactly what Fortunato did to Montresor in order to cause Montresor to enact this most vile crime. The reader does learn, however, that Fortunato easily falls victim to the crime because of his susceptibility to flattery. Ironically, if Fortunato's ego were in check, he would never have succumbed to Montresor's plans.