Each has its advantages and disadvantages in my opinion. IRC is, I think, the most efficient way to communicate with others, because anyone can access it at any time without hassle. No need to add contacts or install any programs - just log in and you're set, unlike Skype. However, Skype is much prettier, has a much shorter learning curve (it took me a while to become accustomed to IRC), and has a bunch of nifty extra features, such as playing games with others or uploading files.
For personal communication with friends and such, I prefer Skype. For the purpose of NIWA, though, I'd choose IRC, simply because I think it's the most efficient way of communication.