Whenever I see pugs their iris is always at the end of the white part of the eye.

Do you mean like this:

http://3.bp.blogspot.com/_Gvk1C-HBfrI/R5NFp9r60iI/AAAAAAAAAg8/A4h1Dmpwvws/s320/pug.jpg

It's just how many of them are, there isn't really a definite explanation, it is like how some people have cross or 'bung' eyes.

Here is what the breed standard has to say about pug eyes:

The eyes are dark, very large, bold and prominent, globular in shape, soft and solicitous in expression.