Issam Lahlali wrote a comprehensive
article in which he analyzes Irrlicht with CppDepend, a Static Analysis Tool for C++.

Of course, the blog post is a kind of advertisement for cppdepend, but it's nevertheless quite interesting, I didn't even know that tool exists. Also, issam seems to come to the conclusion that Irrlicht is quite well designed, so it's worth a read ;)
I've been reading Irrlicht source code recently and really like the descriptions in the code comments and also the fluid layout of the engine. The Drop() design for objects, was an "epiphany" moment; at the same time simple/powerful way to minimize memory leaks. Not to mention the renderer API independence and how that's maintained. (reading on...)