What's wrong with the UX of IRC? IRC is a protocol, not a specific client, and it supports a wide variety of server and client plugins. We had a really nice IRC going productively for a year or two, but we scrapped it because only our software developer used it and other people in the company preferred something else (they used a mix of email, Google chat/hangouts, and text).
The nice thing about IRC is that everyone can use whatever client they like and it will just work. I preferred command-line UX (irssi) because most of my job is based around the terminal, others prefer desktop clients, while others prefer web clients, and all were available for our IRC server.
Our problem was that management wanted everyone on the same thing, and neither side (IRC vs Google chat) wanted to switch (they liked integration with web Gmail, we liked extensibility, and had already built useful plugins, like "run a build"). In the end, they didn't like our "less professional" plugins (Chuck Norris facts, gifs, etc), and we ended up compromising on Slack (we rebuilt some of our custom integrations).
We went through a year or so of adjustment, and now we're reasonably productive with it. However, morale is a bit lower, and I think we're a bit less productive than before, but that's really hard to judge. Honestly, I just wish there was some nicer looking IRC client. I don't like trusting Slack and would prefer the tried and true protocol that we can tweak and just pay for a flashier client for those who like such things. However, it's better than the previous situation where teams just didn't communicate in text because they couldn't agree on a medium, which led to more interruptions than the annoying Slack notifications (non developers tend to always @ the tech lead instead of letting it be answered by whomever happens to be looking at the channel).
I think it's wrong to blame IRC here. The UX of IRC is fine, it just doesn't have a flashy client (pidgin is really easy to use and available everywhere, but it's kind of ugly).