Unfortunately, the IRC gateway causes a bad experience, not so much for the people using IRC, but for the people using the real Slack client. Slack apparently doesn't recognize username mentions ("@joe123") coming from IRC, so everyone needs to add their username to their list of highlight words so that the team members using IRC can mention them.
One could use Bitlbee to connect irssi (or any other irc client) to a jabber server and not only.
It might not be a big deal for you, but when we were briefly using Slack at previous company, I loved that I could just add it as another network to irssi.