I dont want to pass the edges of the iris this question is pretty dumb i know but Im very noob with this but learning rlly fast :ba:
Select only the faces you want to paint and then enable the Face Selection Masking button in Texture Paint mode
hey man do you know how to add a circle to a face? i can do the iris manually too but I dont think is a good idea xD
Various options
Just use the standard brush, set the curve to a sharp falloff and set the stroke to airbrush
Or you could use a brush texture and paint with the texture or use as a stencil (Stencil demo: http://screencast.com/t/sUa95yklt5)
mate I hope one day I can help your projects too thanks