One thing I have noted about irony: In written text, where you don't have cues like tone of voice or body language to detect irony, you usually rely on your the knowledge of the writer: This person does not usually write like this/have this viewpoint, hence it must be ironic.

On an internet forum you usually don't know the person writing (unless you keep track of usernames). And no viewpoint is so extreme or bizarre that there isn't someone somewhere on the internet saying the same thing in all seriousness. You cannot detect irony just by looking at the post in isolation.

Irony can be detected when are familiar with the poster or the general sentiments on the forum, and detect when something is so far removed from the norm that is must be ironic. In turn irony can be used as a marker for group - everyone in the group know what you are supposed to say, an therefore can easily detect when the group members are being ironic. Outsiders will not understand the irony and get confused.

Hence "you have no sense of irony" actually means "you haven't been on the forum long enough to be familiar with the comme il faut viewpoints and language used here. You are not part of the club."