Sounds like a project more suited to an ARM9 style CPU, rather than a micro controller, maybe a dsPIC would be able to cope, but video and image manipulation require a) lots of ram b) lots of cpu cycles.
Most recognition systems the use a visual cue use an adapted neural net to "learn" the associations, some use discreet pattern matching based on a set number of points ala fingerprint, but I think iris will be a tough one for a small MCU as the image cannot be enhanced to the degree that only relivant data is visable as in fingerprints.
I don't know of any neural nets on MCU's but I am sure people must have made some, so a good google may turn up results.
The project would be a simple affair with an ARM9 cpu and a few megabytes of external ram however so thats the way I would go if I were doing it.