Irrlicht is a free open-source cross-platform real-time 3D engine written in C++
OGRE (Object-Oriented Graphics Rendering Engine) is a scene-oriented, flexible 3D engine written in C++ designed to make it easier and more intuitive for developers to produce applications utilizing hardware-accelerated 3D graphics.
"Irrlicht has a cleaner api lower system requirements and works better across platforms than ogre in my opinion"
from question "Crystalspace vs. Irrlicht vs. .....?"
"Irrlicht is no way better than ogre 3d or am not trying to prove that"
from question "Need advice on 3D and 2D C++ framework selection"