Locating the outer boundary of the iris
Hough transform is an edge based operation. For this reason, if you could extract the edges of the outer boundary (together with many noisy pixels) and adjust the radius parameter of the Hough transform, you will be finding the circle of the iris, but not anywhere else. You might want to check:

[centers,radii] = imfindcircles(A,radiusRange)


radiusRange is the parameter you should get right.

However, sometimes for some noisy images, Circular Hough Transform might not be the solution. In such cases you might try Hough ellipses or other methods for locating geometric primitives.