Similar to indirect rays, irradiance rays are fired out from the surface in order to sample indirect illumination. Luxology has both irradiance and indirect ray settings since the two forms of indirect illumination require vastly different numbers of samples. Irradiance caching relies on much higher quality samples distributed sparsely across the project whereas indirect illumination without irradiance caching uses lower quality samples at every pixel. Having two values allows you to easily switch between irradiance caching and traditional indirect illumination without constantly adjusting the number of rays for each sample.