The IRC channels? That doesn't seem an accurate way at all, I know I've never used them. I know there is a way to look at the number of bitcoin nodes. Which would give you the number of machines running a bitcoin client at this moment. Double or triple that and you would have an order of magnitude guess.
I think you might have the wrong idea of IRC there, basically there exist 100 channels on freenode(?) named something like #bitcoin-(1-99) or something like that; when you start your bitcoin client it joins one of these channels at random and collects a list of initial peers for it connect to - this process is known as Bootstrapping. Once it has this initial list is doesn't necessarily need to talk to IRC again since it can then fetch new peers via its peers and so and and so forth, the slight downside to collecting the stats like this is that you can disable IRC bootstrapping in the bitcoin.conf and either enable a static DNS bootstrap (aka fetch a list of peers from a predetermined address) or simply manually add nodes/connect directly to someone.
Edit: To more accurately answer the OP if you wanted to only find a list of addresses that contained at least some currency or have had some kind of transactions in the last x days this could be done by analysis of the block chain, blockexplorer would probably be a good place to start.
