It's a lot easier to write a functional IRC client by glancing at the RFC and banging on a piece of code until it mostly works. For example, http://lists.canonical.org/pipermail/kragen-hacks/2008-Febru... was written that way. Given that the only protocol message it has special handling for is PING, though, I'm pretty sure you could have written it based solely on the RFC.
However, I think it's a big mistake to claim IRC "scales amazingly well". The biggest IRC network today has tens of thousands of users (at the moment, freenode has 64000, undernet has 58000, and EFNet is down in the 32000 range) and the IRC networks are constantly suffering from breakdowns from overcapacity. Compare this to Skype, Facebook, or Gmail, with tens of millions of concurrent users.