Replacing slack with IRC really doesn’t feel like a sincere proposal. Slack has persistent history for everyone, push notifications, image and other file uploads, audio calls, video calls, and screen sharing. And all of that works with no configuration or research.
The only way it’s going to fly is if your organization is entirely hacker types.
I did work at one company where the main means of communication was an IRC server on the intranet. Calls were done through VoIP and file sharing was via the usual shares --- usually the coworker's machine, but central servers were also available. Of course this was a company that specialised in networking and communications products, so perhaps they were more inclined to do such a thing (and we "dogfooded" all of our products too), but nonetheless it was a great experience.
The only way it’s going to fly is if your organization is entirely hacker types.