I had been to some NY local meetup like this before. That didn't work out well. AFAIR, probably two or three devs would be there to help answering questions like "how do you do this in Django" sort of thing, since a lot of people found face-to-face help valuable.
So what didn't go well even when there was a low volume of seekers? Say there was only five people seeking help, and two volunteers to help. If someone had a "good" question, the helper could be stuck by the seeker's side for a while. 1-1 model would be nice. If you were a frequent seeker, you could build a bound/friendship with a helper, so that helper would more likely come to help you since he/she would be familiar with your questions already. See the problem there? Then for the newcomers, they came in finding themselves lonely in the corner, desperately trying to get someone to help. Eventually I found some helpers like to spend time have causal talk with certain seekers, while spending much less time on others, which was very unfair to those who came to seek help.
IRC has both noise issue and focus issue. Too many chitchats going on in the background, and too many people trying to answer the same question (which is a great to see people are happy to help), but all end up arguing. So sometimes, I just message privately to keep the conversation healthy and productive.
Either way, for quick questions like "how do you do this" or "what does this error means", IRC/Slack/Discord/Stackoverflow works fine. Face-to-face is a better option for as long as there is an adequate helpers available and when helpers are discouraged from causal talk to avoid the Dining philosophers problem.