IRC is old chatting protocol with advantage of simplicity. The protocol are lines of text with maximum length of 1024 characters (buffer size).
Maximum users on server are usually few thousand but there is option to connect multiple servers together.
There are official specifications (RFC 1459, RFC 2813 and IRCv3) but I will write it here in more human-friendly format.
I split the protocol into 3 parts:
Encryption
It is usually sent in plaintext but those days, most server support SSL....