Firstly, you’ll need to download and install Empathy and the relevant libraries for irc connection:

sudo apt-get install empathy telepathy-idle

Start Empathy (it should be in the internet menu), and then press F4. Fill in the resulting box as follows:

If you enter a password it will automatically /msg NicKServ each time you connect to authenticate you.

To join a chat room, go to Room and Join New. You should get something like this:

Select the account you set up earlier (mine was called irc-darkmyst because I use more than one irc server). Add the room you want to join, and click on Connect. You should then be able to chat on irc as normal.