Ive found this very brilliant bit of work by Florian Boesch on Codeflow (http://codeflow.org/entries/2012/aug/25/webgl-deferred-irradiance-volumes/) which includes source code. It’s webGL but the outcome is awesome, and could easily be ported to full open GL aswell. The above link has a brake down of all the components,implementation. Very nice stuff.
Video: