A lot of people may think “old-school” when you mention IRC to them. This way of communicating online first came to popularity in the late eighties, and got pushed under the radar for a little while with the emergence of social networks, Skype etc.
IRC has seen a resurgence in popularity in the last few years, especially among developer and FOSS (Free and Open-Source Software) communities.
WordPress uses Freenode, an IRC network dedicated to FOSS communities. To connect with Freenode, you will need to either download an IRC client, or use a web chat client-
Te choice of which IRC Client to pick heavily relies on which platform you are on (Windows, Mac etc), and there are too many option to list, but the most widespread ones are mIRC for Windows, irssi for linux and Colloquy for Mac OSX. There are many others of course, and it helps to shop around for the client that best suits you.
Alternatively, you can use Freenode’s web chat: http://webchat.freenode.net/. This requires an ajax-compatible browser.