Sorry to be devil's advocate here. I have good enough experience with IRC and Slack (used it at places I have worked at).
I would suggest to stay away from Slack. The reason Slack became popular is because of very low learning curve else it works on nearly the same principles as of IRC (sometimes we call it IRC with better UI).
Yes it is filled with features, actually it is crowded with features, and that is what makes it evil in the long run. People will start doing stuff which they don't even need, just because it is easy to do (mostly one click) and then there will be garbage all around, broken flows, useless integrations because everyone will have their own ideas. People like the offline feature but it is another evil. In Slack, when you come online after a small break, you will find plethora of messages flowing in sometimes, because no one cares whether you are online or offline. This develops into shortcut habits of sending messages to people now and then. The loop of offline messages created this kind of momentum for slack, and made it a user habit forming product. This gave it the ability to leverage on the FOMO of people.
For a new user many things are hard to on IRC, and actually it is good, because then they are done only when there is a strong need. This keeps noise low. The list of things I see mentioned above in favour of slack, are features it has in comparison. Unless lack of such features is creating a critical failure in communication and collaboration, it's better to delay the idea till a pressing need comes.
Slack doesn't makes you more productive as compared to IRC, it only makes everyone feel more connected.