I would, at least initially, advise against Slack.
I constantly hear people decry its growing use in open source communities for a mix of two reasons. The first seems to come from strong IRC proponents, with the general complaint of "it's not open," and getting into the whole "your data is not your data" idea, especially with recent changes to Slack with regards to privacy. The second seems to come from the inability to really get away from Slack. Their notification system is pretty rudimentary, in that there's not much fine tuning to be done and most people I've talked with are either overloaded with things to care about or mute everything and only manually check channels. There's an additional problem in that you can never fully be away from Slack. Their "Do Not Disturb" settings can't stretch for more than 24 hours (something they do not appear eager to change as they're asked about this quite often), so you need to go out of your way to do something about the weekend to get some time away from work.
However, I'm not really familiar with the initial problem of people joining IRC. Rather, one I'm familiar with regards to IRC is the problem of replicating things like Slack—or at least the ability to receive messages while offline—such as using/buying/hosting an IRC bouncer. Can you speak more about what brought you to propose a change here? I wonder if this is something we can just solve while continuing to use IRC?