How to overlap multiple textures with restricted Opacity

I have uploaded 4 textures to help convey what I’m trying to get across. The textures are for skin, sclera, iris, and eye lid. I want to overlay all the textures onto the skin, the iris on top of the sclera and the eyelid on top of the iris. However, in addition to that, I want to know how to restrict the opacity of the iris so that no part of the iris appears outside the bounds of the sclera. I have a vague idea of how I want to achieve this but I’m not 100%. If anyone knows, I’d greatly appreciate it.

With regards,