You have to install it as an app on slacks side. Some employers don’t allow that.
It's not IRC, but there exists a working libpurple plugin that you just auth as with your normal user account:
https://github.com/dylex/slack-libpurple
I just use it with Pidgin, but there are curses clients too (I'm aware of bitlbee, there are probably others) if you prefer the IRC-style experience.