Although the short story "The Cask of Amontillado" is not typically thought of as being humorous, it certainly has moments of dark humor. This issue concerning the masons is one of them.
When Fortunado makes the "grotesque" movement that Mostresor does not comprehend, he was showing the sign of "the brotherhood" of the Free Masons, an old secret society. So the irony is that Fortunado is referring to the secret society and Montresor pulls out "a trowel" showing that he too is a mason. The mason that Montresor refers to is the straight definition of someone who builds and works with masonry.