You didn't attach an image, you attached a screenshot so I can't run any code on it. Anyway, here's how to do it (untested)
% Invert the image so you have a white ring.
maskImage = ~binaryImage;
imshow(maskImage);
% Fill the image so you have a white disk.
maskImage= imfill(maskImage, 'holes');
% Mask your original binary image.
binaryImage(~maskImage) = false;
Or, if you already know a point in the iris, you can use that to make a marker image and then call imreconstruct().
markerImage = false(size(binaryImage));
markerImage(row, column) = true; % Put a dot where the iris is.
% Extract only the blob containing the dot.
outputImage = imreconstruct(binaryImage, markerImage);
