IRC dates back to 1989. I myself first compiled a client on a HP-UX 9 system and connected in 1993. IRC was relatively big because it preceded the web; it was one of the few cool, interactive things you could do over the network. That and some early networked games. MUDs, shooters like Hunt (1986), XPilots (1992?)
Even before IRC there was "talk" which let you chat with anyone logged in to the same host (don't recall if it supported network chat). Remember in those days few people had a computer on their desk, instead they had a terminal that connected to a central host.
Yes talk did work over the network. The programmers that wrote it, though, made fundamental mistakes in it. They sent raw C structures over the network without marshaling, oops! So talk didn't work very well. The issues were addresses in "ntalk".
