Irony is a rhetorical device, literary technique, or event in which what appears to be the case, differs radically from what is actually the case.
The chief priests and the scribes do not realize that Jesus has the power to save himself but on account of his messianic mission he chooses not to do so because he wants to save others. The taunt of saving others thus actually expresses a truth. If Jesus was to save himself from God’s wrath at human sin, then he would not be able to save others.1
31 So also the chief priests with the scribes mocked him to one another, saying, “He saved others; he cannot save himself.