Irc stand for Internet relay chat. IRC was designed for group communication. IRC went famous and really started to expand when it was used during media blackouts in Kuwait and the Gulf War.
Anyway today there are 100’s of thousands of channels, most of these can not be picked up by search engines and if someone does not tell you, or you dont find a link to one, you wont even know it exists. Just like websites but the difference is that IRC can not be indexed. Only the files on IRC can be really indexed.
There are ways to find out what’s going on in the IRC world through places like Searchirc.com but you really have to know what you are doing to find what you are looking for.
Irc remains a huge part of the deep web even until today.