Don't exactly know what iATN chat room is, but IRC information can be obtained from: http://www.geocities.com/~mirc/
It is a chat client for a network of chat servers in one simple to use program (maybe not as simple as AOhell's Instant Messenger, but relatively easy to use) You can get information off this page on how to set it up and configure it.
I use an EFNET server (there are a few different ones) and usually keep a channel called #zeno'z_kitchen when I am on and my nickname is Stevenray (after you get it set up do a /whois to see if I'm on.
Maybe I ought to start a Z channel on it somewhere... More on that later I guess
It would be kinda like this list but wouldn't be able to peruse old topics like this message board...But it is realtime chat!