IRC is the internet relay chat protocol & program suite.
I think this is the oldest one that'll still work on modern servers. The source that I usually look for is ircii-4.4
As far as I'm aware, this is the oldest version of the IRCd. The protocol is out of date, no current client can talk to it, other than the bundled one. I think it may be wise to try and run it on 2.11BSD or similar. See http://www.irc.org/ftp/irc/server/Old/