Lighting an Iray scene is a combination of the scene lighting and render settings which act like a real world camera & lighting setup. If you find the scene is to dark it will either be the amount of light or the the Render settings you need to adjust to bring it up further.

Adjusting the luminous flux is the obvious way of boosting the light in the scene, but it's easy to overdo and putting a large bright light on your scene may effect the realness of your render. You can also change the light measurement system for Luminous flux from Lumens to watts with which we all seem to be a bit more familiar with due to home light bulb measurements.

Another thing to adjust is in the Render settings and is the Tone Mapping panel, here you will find many normal photographic type controls Iray needs adjustment the same way a DSLR camera does. Settings that work great in a cosy fireside nook of an old house are just not going to work on bright sunny day at the beach the same as a real camera.

The main option to look at is the Exposure setting: "Each increment or decrement in this value corresponds to halving or doubling, respectively, the effective exposure as expressed in the resultant change in the Shutter Speed value. Higher values yield darker images, and lower values yield brighter images."

Adjusting this setting is basically adjusting your virtual camera to be more or less sensitive to the light, just as you would with a real camera.