Many projectors today have irises in them. The type varies between dynamic and manual. The dynamic version moves as the picture brightens and dims. Manual irises stay fixed at the setting chosen. Some projectors will have both while others will have only one.
Manual irises are a huge benefit when there is excess light output. It can be used to match a projector to a given service in this situation. Sports for example looks best relatively bright while movies generally look better at lower levels. Reducing the light output should be considered for movies when a projector can go brighter than about 15 fL with a given screen. As the lamp dims with time it can be opened to keep the image looking brilliant.
Dynamic irises are more variable in their success. These can be noisy and a point of failure. When implemented well they are not a distraction and make the blacks darker. Done poorly and you can see an image suddenly brighten or dim as the scene changes. The brighter the image from the screen the easier this effect is to see. The best implementations I have seen are Sony projectors. Other manufacturers tend to have mixed results.
Another benefit of an iris is that when it is placed inside the lens it will reduce aberrations. Unfortunately, many manufacturers take the less expensive option and attenuate the light outside of the lens.