InternetRelayChat is one of the older systems for chatting online - it is oriented towards groups of people rather than towards pairs of people as the Instant Messaging systems are.
If you want to use IRC, there are many resources to help you. Find some at http://directory.google.com/Top/Computers/Software/Internet/Clients/Chat/IRC/Help/
If you want to write an IRC client, or some other program which interfaces with IRC, you'll want to look at RFC 1459, which defines the IRC protocol. There are almost certainly decent, open-source standalone libraries to help you, as well.