A decent 5 mega pixel will do. The key is that it be macro. You will also need a side lite ( I prefer LED light). It's all about getting good illumination without glare.

Hi
Many people experiment with cameras and lights to get reasonable iris pictures but not many get good images and it takes many tries to get it right.
Iridology cameras are getting cheaper and better all the time
Although there have been some problems with Chinese units there is a Australian made USB iris camera that connects directly to your USB port. Have a look here http://www.iridology.nl

In the long run it is much easier to get something that works all the time then having to experiment each time you want to make a picture. It is NOT the MP that counts but the lens and the lighting even 2Mp gets you great images.
Good luck
Leo