Yeah, IRC bots are rarely the method used for viewbotting - if you look at viewer lists, particularly during massive viewbots (like the 10k+ ones) you'll usually see significantly less accounts than viewers; generally like 200 which are mostly people coming in to see why this guy playing tf2 has thousands of viewers. It's especially easy to see through a separate IRC client which will tell you how many people are actually in the channel. Granted, you'll probably have at least some viewers who don't have an account on 100+ viewer streams, but the majority (I'd say about 80%, maybe less on something like a League or SC stream but still significant) are going to have accounts and chat. Cobra is more correct in that it's easier to spoof requests to the site, almost like a DDOS lite, and it's the reason why @BotDetectorBot judges viewbots by a large number of unregistered viewers.
Granted, you can use a bunch of IRC bots, and I've seen it done, but it's much more intensive (as said above you need to reg accounts, get auths, etc) and as such I've only seen it limited to like 200 views or so, maybe a few more. I'm actually rather certain that's the method that Tacos kid uses, though; his views include a lot of real accounts.
As a side note, the kid viewbotting tf2 streams lately always uses an account with a variation of the name furaffinity (no, I'm not kidding). So far I've seen 3 or 4 accounts of his and he's too retarded to use a different alias apparently. If you get botted, look for him, ban him, and report him; point out that it's another alt. I don't know if twitch will IP ban anyone but I did say that it was his 4th account last time I reported him.