How to chose right lens for concentrating IR signal?
You'll need to know the LED's beam angle and wavelength, the distance to the wall and the spot size on the wall, and then you can get ahold of the folks at Fresnel Technologies for help.

They make plastic Fresnel lenses and can help you out with the optics and materials to use for your application.

I used them to supply me with lenses for a long-distance PIR project and was well satisfied with their performance. 

BTW, here's some interesting reading.