Its probably noisy because the light levels in your scene are still low and Iray was stopped before it had enough time to build up the samples the scene needs.

You don't need to use an enclosing box to stop that overall light. Just switch the render settings to 'Scene only' and the dome will turn off. Better for me is 'Dome and Scene' with the dome intensity turned down. I tend to use spotlights instead of point lights and give the spotlights some geometry.

regards
prixat