The irc scheme is defined in the Internet Draft at
https://www.w3.org/Addressing/draft-mirashi-url-irc-01.txt (which
was never approved as an RFC). Such URLs have the form
irc://host:port/target,needpass
and are retrieved by opening an IRC session using the
function specified by url-irc-function.
The value of this option is a function, which is called to open an IRC
connection for irc URLs. This function must take five
arguments, host, port, channel, user and
password. The channel argument specifies the channel to
join immediately, and may be nil.
The default is url-irc-rcirc, which uses the Rcirc package.
Other options are url-irc-erc (which uses ERC) and
url-irc-zenirc (which uses ZenIRC).