If you possess some older Genesis 2 series characters for Daz G2 you may find they dont come with materials for optimized for iray rendering with only 3delight materials included (in my Library there is actually a iray optimized g2 mat -but I'm not sure if is in the current DS)
To Convert those materials to iray, you need to select the materials surfaces - start by selecting G2 and then the surfaces of the Skin.
Then, under Shader Presets>Iray>Daz Uber, while holding down the Ctrl key click on "!Iray Uber Base"
On the pop up, Material Preset Load options, select "images" and set to ignore. The shader base preset will then be applied while keeping your textures in place. There are other shader presets you can apply to other surfaces such as "water thin" for the eye cornea
There will be tweaking to do as all textures are different, maybe start with the glossy and bump settings. It helps if you are able to have an iray preview running in the viewport.
There is also the Iray Smart Converter and Advanced Skin Managers for Genesis 2 product from V3Digitimes
I find it does a great job from start to finish and can really speed up conversions while giving you control of how you want your skin to look at every step, the instructions are also embedded (in the pop-ups) to guide you through the process giving clear instructions on choices you might make. You can also use this to tweak an already converted skin to your liking.
Here's a quick overview posted by V3Digitimes on YT