The name of the tool escapes me at the moment, but there was an IRC-type tool running on the IBM mainframes I used at university in 1985. IRC dates to 1988, and Zephyr 1990, making them a bit late on the scene.

Ah, a bit of googling shows it was relay. Of course, for one-on-one instant messaging, Unix 'talk' goes back much farther than that (early 70's), along with similar tools on other OS environments.

--Dan.