As far as I can tell, Slack has an order of magnitude more users. They were at 8m DAU back in May: https://techcrunch.com/2018/05/08/slack-hits-8-million-daily...
IRC was in long-term decline even before Slack: https://royal.pingdom.com/2012/04/24/irc-is-dead-long-live-i...
So my best guess is that Slack has at least an order of magnitude more users.
But users is not exactly the metric I'm thinking of. It's when people say, "We need some way for us to communicate," Slack is the popular option. Even though I'd rather not use it, I'm in 8 right now. If you look through this thread, you'll find plenty of people concerned about how prominent it is in the open-source world, IRC's home ground.
> IRC runs everywhere and does not need a fat browser or a RAM-and-CPU hungry application to enable "chat".
I get this in theory. But RAM and CPU power are fantastically cheap. Conserving them to get a worse user experience is optimizing for the wrong thing.
> but Slack sucks in many ways
Sure. There are no perfect things. But Slack doesn't have to be perfect. It just had to be better than the competition. And for most people it manifestly is.
In which world does it beat IRC? Number of users? That's not even sure, since there are dozens of thousands of IRC servers worldwide. And that's not even counting the private, self-hosted IRC servers.
IRC runs everywhere and does not need a fat browser or a RAM-and-CPU hungry application to enable "chat".
As for the user experience, this would merit a whole post in itself, but Slack sucks in many ways: threads are completely unusable, channels are hard to discover unless you know about them, and now Slack makes you pay for searching thru your past messages.
I'd say there is a lot more to this topic than "Slack beat IRC".