I just did a very quick and dirty test using hardware instancing (based off a D3D sample - completely non-Irrlicht) and I get:
1 million cubes @ 7.3 fps
100,000 cubes @ 50.9fps
Peak memory usage is 77MB.
Stress Test
Irrlicht Demos: http://irrlicht.sourceforge.net/forum/viewtopic.php?f=6&t=45781
FYI: I just tried a DX11 card and got 26fps for one million instanced cubes.
Irrlicht Demos: http://irrlicht.sourceforge.net/forum/viewtopic.php?f=6&t=45781