I think that the twitter and irc people are two very different categories of netizens that almost don't overlap.

I can anecdotally disagree with that...in fact, the only reason I got a Twitter thingy after resisting for about two years is because the crowd in my IRC hangouts incessantly refer to "tweets."

Twitter is certainly not a replacement for IRC in these circles either, though, merely an extra tool. It is not well-suited for it, not least because support for anything like real-time conversations between multiple people is almost completely absent -- which I think the correct design decision.

Bizarrely, some of the non-IRC people are trying to sell Twitter as some kind of a chatroom, see for example this recent TIME Magazine article:

http://www.time.com/time/business/article/0,8599,1902604,00.....

Edit: tautology.

I've got at least 10 friends who use both, but what I've mainly found is that my old friends who used to IRC back in the 90's and have since left, enjoy twitter. They like it because it's like IRC, but you don't have to give it 100% of your attention to keep up with the conversation. We refer to it a slow version of IRC where the channel never closes.

I use both. In fact, I can think of at least 12 people off the top of my head who use both. They're not mutually exclusive, after all.