IRC does not have this. I've to connect to your server, or you to mine. This doesn't scale as an email/IM replacement; it only works for rooms with a certain usage/target.
The huge problem with any replacement (aside from federation) is the network effect. No matter how good your solution is, you need critical mass for it to pick up, and no huge company is going to invest into getting users into a federated network which they won't control. There's no profit for the company.
IRC most certainly does have this. All larger IRC networks are exactly that: Networks of federated servers. It's been like that since the early 90's, when it was an absolute necessity as none of the ircds could handle all that many individual users.
Indeed, it’s the main catch. If at certain point ago critical fraction of people (‘everyone’) all were using X for what we currently use email for, be X IRC or anything else, then X’d probably be the today’s email.
Services such as the aforementioned Slack seem to be going maybe the only pragmatic ‘email disruption’ route. In corporate world you can have the central authority say “everyone use this or you’re fired!” and thus bypass the network effect barrier.