[queball]

Traffic monitoring would make it obvious that there's a server, and also very obvious that it's a chat server. Just using ssh tunnels, further traffic analysis would reveal who talks to who just based on packet timing. The spy would have everyone's IP, and could intimidate any number of them into helping him monitor the server. Or the server could be taken over physically, through legal or illegal coercion.

The internet is monitored by government and private institutions. No-one really cares if you set up anonymous chat with a few friends; after all, you could just go and talk to them in person if you wanted to. But once you start interacting with the outside world, with other underground organisations, you have a weak link, and the whole structure can come down at any time.

Cyberterrorism refers to using the internet itself to attack people, abusing security at a technical level, from destroying others' use of the internet to more insidious cracking. Setting up a private IRC server is perfectly legit and does not threaten security.