Irssi is a modular IRC client for UNIX that only has a text mode user interface. Irssi isn't really even IRC specific anymore, there's already working SILC, http://www.silcnet.org and ICB, http://www.icb.net modules available.
The irssi homepage is http://irssi.org where more information on this program and the latest release, scripts, themes and plugins are available.
Here is an example way to start the program to connect to the european undernet server:
irssi -c eu.undernet.org
Once the program is running, you can join a channel: /join #LinuxQuestions
change your name: /nick LQwriter
leave a channel: /part #LinuxQuestions
close a chat: /window kill
and close the program: /quit