What is IRC?
IRC stands for "Internet Relay Chat". It's basically a chatroom service. There are many chat servers, hosting different channels. You can think of each channel as a separate chatroom. To join the channel, you need to know its name and the server it's on.
You can find out more about using IRC here.
Hackspace IRC Details
The Hackspace channel is #nottinghack on the chat.eu.freenode.net server.