[ajb]

The Iriscode (if it's the same as Daugman's iriscode) has tested so far to have 249 bits of entropy, or over 10^74 combinations [1]

So even with the birthday paradox you'd need 10^37 people before having a good chance of a collision, which is rather more than we are likely to have in the next few centuries.

Of course, it's possible that there are some subpopulations who don't have this amount of entropy in their irises, most obviously the small number of people who have a birth defect such that they are born without eyes.

[1]https://www.cl.cam.ac.uk/~jgd1000/binomdata.html