Comparing IRC vs Slack maybe made sense when Slack was very young, and it was like buying a Honda Civic vs a Ferrari, where one is obviously nicer but the other still accomplished the same thing with less frills.
These days it's the choice between buying a Honda Civic vs a Boeing 747. They both get you from one place to another (communicating between employees) but that's where the similarities end now. Slack has way too many API integrations, historical search, features for everything from compliance message archiving to embedding images to dumb GIF searches that people seem to like. The fact that it has irc-like channels and chatting isn't really the point of the platform anymore.
Yes you could code this all yourself using IRC bots, but at that point IRC is no longer free, it's costing you time. If you get to that point, Slack does it better and cheaper.