The Avatar List (Radar) is a tool that lets the user see a list of all Avatars within range (draw distance and same sim), as well as providing access to various Avatar-related features, such as Profiles, IMs, TPs, and moderation tools all in one window.
IRC is an acronym for Internet Relay Chat, a chat protocol that is very old (ca. 1988). IRC could be compared with group chat, but without the chat lag.
IRC has no relationship to Linden Lab and is a completely separate chat protocol.
For more information about IRC, please see this article: http://en.wikipedia.org/wiki/IRC
IRC allows users to join the Phoenix help channel as an additional method of getting support.