These are different things. Signal is designed as a secure, more robust alternative to SMS/MMS.
IRC is not even close to filling that role. It’s not more robust nor more secure. At the same time it does several things that Signal doesn’t. Signal, as a messenger, is not a “chat room” or “social network” in the same way that IRC, Matrix, and even Telegram function. There are. I thousand member rooms you can join. It’s meant for secure, small group, communication.
Maybe that will change and they’ll broaden, but I would rather see them keep to their niche and do it well. I have no problem having one messenger app and another for “communities” of sorts.
Failing that, certainly Matrix is the best option?