IRC, or Internet Relay Chat, is a feature available to all TerraNovians. IRC is accessible by clicking the IRC link on the header of the main TerraNova website.
Upon opening the IRC page, you will be prompted to enter a username. Usernames can be registered to be reserved, so if your's isn't unique, it's probably already been registered. If you connect to IRC using a registered username, it will be changed within 30 seconds due to a failed login. After you've connected, you'll see all in-game chat messages. If you type a message, it will get sent to the server.
IRC is not limited to just the in-web interface TerraNova offers. You can use other IRC clients such as Nettalk, mIRC, ChatZilla, and XChat. If you already use another IRC client, you can use the server irc.mibbit.net (port 6667) and join the channel #tngaming.