Cirrus, as well as being high altitude clouds is also my high-level Nuaj wrapper also available at the same SVN repository as Nuaj here.
It's loaded with cool stuff like a FBX scene importer that allows you to load any FBX scene from your favorite 3D package like 3DS Max and Maya into a Cirrus scene that can later be saved as a unit using the (quite lousy) Cirrus proprietary format.
Cirrus is not a 3D engine filled with humongous amounts of code and 3rd party libraries : as for Nuaj its sole purpose is to ease the development of 3D projects so you can quickly come up with a 3D application that fills your needs (and some polygons at the same time, mirf hirf hirf 
).
The architecture in Cirrus is quite simple and is organized like this :
Structures
There's nothing much to say about the Renderer and the Pipelines, a Renderer is (usually) a singleton (although you can create as many as you like) that contains a collection of pipelines which, in turn, contain a collection of RenderTechniques.