Beginners Guide To IRC (mIRC style)
Based on the chatting client, this is a good explanation on how to get started, easily. Contains plenty of other related links.

Client-To-Client Protocol (CTCP) Documentation
Specifications for CTCP as well as DCC protocols which are now standard supplements of IRC.

Internet Relay Chat (IRC) Help
Nearly 800 helpful files including FAQs, primers, technical guides, clients and scripts, and server lists and maps.

IRC Command Cosmos
Advanced reference of IRC commands.

Most updated RFC1459
IRC Protocol

Undernet FAQ Website
UnderNet's IRC-related FAQ

Undernet Routing Committee
Resources for Undernet server operators.