As far as I've been educated about IRC, the very history of the internet and IRC are synonamous <mind my spelling> with each other.
The internet originated and evolved around military use. IRC would have been used by the military since the very beginnings of the internet.
IRC was created by Jarkko Oikarinen in August 1988 to replace a program called MUT (MultiUser Talk) on a BBS called OuluBox in Finland. Oikarinen found inspiration in a chat system known as Bitnet Relay, which operated on the BITNET.
Reference:
http://www.mirc.com/help/jarkko.html