Sometimes when I am on the road or do not carry my big camera with me, I use my iPhone 4 to take pictures of the Iris. 

I place a magnifier with a light over the lens of the camera. This serves two purposes. One, there is enough light to capture a good photo and secondly, enough magnification to provide sharpness without loss of detail due to pixelation. 

Here are some photos that illustrate the process.