IRC stands for Internet Relay Chat. This is the protocol (type of system or set of procedures) used to run most internet chat sites. It allows for the "back and forth" type of "realtime" discussions between people in a chat room situation.
The term "IRC" is usually used to refer to chat networks in general. In the case of the ICQ chat network, the actual Name of the chat network is IrCQnet.
("ICQ" is actually the instant messenger program, and is not the same as the IrCQnet chat network. Although the IrCQnet chat network is featured on the ICQ website and can be accessed from there, the chat network is a separate entity.)