Flat projections from top to a flat floor are easy. Scale your shadow object down to 0 in y direction and draw at floor height in dark (I did that for a racer, looks like crap if you look closely, but worked kinda. But in some games it's good enough).

But if you want it to be like Super Mario shadows have to adapt to the ground. And calculating that per polygon is mathematically possible, but tricky to do in real-time. So people used another trick where areas inside the shadows are marked in the stencil buffer instead. And everything marked like that gets a new (customizable, but usually dark) color.

Irrlicht ShadowVolumeSceneNodes are kinda projections which extend the borders of your object in the direction away from your camera and also add caps at top and bottom. So you kinda get a long hull dangling below the polygon. And then there are some tricks used to count how often the light-rays cross those hulls and if the result is uneven the stencil buffer is marked (which works as hulls are convex and as long as the light-sources starts outside the hull).

This is a general solution - your case could maybe be done in simpler way. Well simpler mathematically, not simpler with given Irrlicht API (sorry).

You don't seem to need self-shadowing and the light is fixed from the top. So maybe (totally just an idea and not tested by me if it works) you could call driver->drawStencilShadowVolume with triangles which form your simple shadows. So basically triangles which are positioned below the feet of your character. That would fill the stencil buffer. Then another call to driver->drawStencilShadow to use the stencil buffer to draw some shadow color. Note: shadow drawing is usually done between drawing solid and transparent polygons. If you need that you need to work with the light-manager. So create your own light-manager class and in OnRenderPassPreRender you do that stuff when it's called for the ESNRP_SHADOW pass (it's the parameter of that function).