One (of the many) problems with using IRC as a signal is that it's biased towards people with large chunks of predictable free time.

I spend a lot of time communicating with people online, but IRC isn't a channel I use. The problem is that I have a family that naturally consumes a lot of my time. That means I don't control when I'm available to communicate. I get a decent amount of total time, but it's in bits and pieces spread throughout the day (mostly early morning or at night).

That's perfectly fine for asynchronous communication like email, SO, here, etc. But it doesn't work for synchronous things like IRC.

So, using "are they on IRC" as a hiring signal, I believe, effectively punishes people who have families, busy work schedules, or otherwise complex lives. In other words, I think it ends up favoring affluent young people. That's not something I'd personally be keen on biasing for in my hiring process given how much of an advantage that group already has in technical jobs.