Irssi is an easy, plain-text IRC (internet relay chat) application. Using it is simple; start a terminal.
Typing just irssi would start irssi but even better would be this:
irssi --nick seth --connect irc.freenode.net
which starts irssi, assigns you the nickname of seth, and connects you to the internet relay network belonging to the fine folks over at freenode.net
98% of the irssi window is the text chatter happening on the server.
Look at the bottom of irssi for where you enter commands.
To join a channel:
/j #channel-name
To private message someone:
/msg seth hi, i'm on irc! let's write some code!
And so on. To log off of irc:
/exit