You might want to visit this site to see the breed specifics for Irish Setters.
http://www.dogbreedinfo.com/irishsetter.htm
I personally don't think that eating poor quality dog food would stunt a dog's growth to that degree but I could always be wrong. My thinking is that the small size is genetic and maybe the dog didn't come from the best breeder. That all said, she has a home now where she is loved, so you just go from there. Maybe you could coax your uncle to try giving her some better food but suggest it nicely.