I've been using irssi to chat over IRC for a while, so when we started using Slack at work I was delighted with their IRC gateway.
To add a connection and make it persistent is simple enough:
/network add -nick <nickname> slack
/server add -auto -ssl -network slack <account>.irc.slack.com 6667 <password>
/save
With others using the Slack app, I was getting a constant stream of mode-change notifications as their status changed from away to active and vice-versa. To stop that from happening, as it turned out, is easy:
/ignore -network slack * MODES