Or one can use bitlbee, which has been around forever and connects an IRC client to anything that works with libpurple. The full list of what it works with is here: https://wiki.bitlbee.org/

Matterbridge seems interesting as well but I haven't had time to play with it yet. https://github.com/42wim/matterbridge

In view of those more-complete, non-proprietary solutions, you have to be a chump to pay for a Slack IRC bridge.