Mattermost is a oppen source alternative to Slack, a well known proprietary business communication platform developed by American software company Slack Technologies. Same as Slack, Mattermost offers the user many IRC-style features, including persistent chat rooms (channels) organized by topic, private groups, and direct messaging. But it includes many features you are missing in IRC like media integration and bouncer-less permanent accounts. Main difference to Slack is the open source licensing AND the factor of self hosting. We always know where our data is! If you need more information about Mattermost, check their web presence: https://mattermost.com/
As nothing in this world works without rules, we made some for Mattermost too. Read them and follow them. Otherwise the banhammer could hit you.