yeah I've seen Irrlicht prefixes used somewhere else (also in other engines). I think irrlicht-style is really good idea. And yeah the prefix is usually used both in filename and in class/struct name. Why using different names that can create confusion? That way if you are searching for ISceneNode you need to look at ISceneNode.h.
OpenGL is not hard. What you have to do is just explained in specifications. What is hard is dealing with poor OpenGL implementations.