A bit of a clarification might be in order. You don't need irc to get this worm. The irc part is probably in because this is one of those worms that turns your PC into a "zombie" and then makes it connect to an irc channel every day or so to see if there are "orders". (you won't see this ofcourse, much less do you need mirc installed for the worm to do this) Then when the worm creators think they have enough zombies, they'll issue an attack command on their mirc channel, and at a designated time all zombie PCs will ping-flood a major website. This is what a DoS (denial of service) attack is. They sometimes shut down major websites like google, yahoo or amazon in this manner (after trying to extort money from them first). If you have the worm, you'll be one of the attackers without ever knowing it.
With this particular worm, there's actually three groups competing with each-other to get as many PCs infected as possible. They started writing these worms as soon as the "bug" (buffer-overflow flaw they use to get you infected) got made public. One of the worms will even delete the versions from the other groups if it finds them on your system!
But they're kinda badly written, because the most prominent one makes your PC reboot, which is ofcourse stupid. The whole point of the worm is to sit there without you knowing it and wait for orders, and rebooting the infected PC totally blows its cover.
Also, this is not one of those virusses where you need to click on an executable attachment or something to get infected. As I said earlier, it uses a buffer-overflow flaw. Buffer overflows are by FAR the most prominent security flaw that can be abused to get control of a computer. It's quite technical, but the basic idea is simple, so in layman's terms: say you have to login and write your username, which has a "20 characters max". Anytime the program does not actively check whether you indeed wrote less than 20 characters, we have a buffer overflow weakness. Whatever you write after the 20 characters, will overwrite the actual program that is running! So the worms connects to your PC, overflows the said login and adds code that makes your PC download the worm from the internet. Bam, you got infected without doing anything.
What started this was microsoft who announced: "in windows2000 there was a buffer overflow error in this insignificant little add-on code, so here's a patch". Ofcourse most companies don't patch that quickly, and microsoft basically told the virus-writers how to abuse their OS :P. (also, if you don't have win2k, don't worry about this virus)