Fortunado’s name is ironic because he is not fortunate—he gets murdered!
Montresor and Fortunado have apparently been acquaintances but not close friends. Fortunado has done some insult to Montresor, and he is apparently not at all aware of it. Still, the injury is enough that Montresor has decided that he needs to get revenge, and has carefully plotted Fortunado’s murder for some time.
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will.
The name Fortunado implies someone who is fortunate. The name is ironic because poor Fortunado has the worst of luck, having done something to annoy the insane and homicidal Monstresor. Fortunado also happens to be drunk, and therefore has no way to defend himself.