You can use mIRC to chat on LAN and without an internet connection. A couple of things to remember....

1. You'll only get to talk to others that are on the LAN.
2. You still need an IRC server, in this case I reckon Dreamforge would do the trick as it is free but you'll need knowledge of how to configure the ircd.conf file which is what is required for the server to run correctly.
3. Goes without saying but the other users need mIRC or another chat programme too.