Yes, they can be very intelligent. I have a pure bred Irish Setter and he is very, very smart. Quick to learn and obey. But he was trained early on - and that's key. This breed's personality can be very "rollicking" with a puppy-like temperament the first several years, so training in the first year, preferably the first six months, is important to make the Setter more manageable. They are lots and lots of fun, but can be hard to control without training. They can also tell if you're the type of person they can dominate - he will become your alpha, but only if you let him. My Setter thinks he owns me. It is quite cute and not at all a problem. My husband, however, has established that he's in charge and it's a different dynamic (though still adorable). Between the two of us, you can really see the difference in how an Irish Setter will behave when he thinks he's the one in charge, and when he doesn't.
My Setter was bred with part show lines and part field. Our breeder specifically breeds all her Irish Setters from part field and part show lines, for the look of the show dog and the intelligence of the field dog. So I agree with some of the other commenters here that the intelligence of an Irish Setter really depends on the breeding. The breed was originally created for hunting birds, but when Nixon had one for a pet in the White House during the 1970's, the breed became extremely popular, and unfortunately their breeding became out of control, especially as the show look was more emphasized, resulting in very unintelligent dogs. Now that the breed is less well-known, I highly recommend you do your research on the breeders out there. Don't just go for a backyard breeder who's bred their Setter just to make money. Look specifically for a breeder who has been careful about the lines they breed from, not just for the show look, but for intelligence. That really makes a huge difference in the intelligence of your Setter. This breed is a good example of what can happen when breeding is done responsibly. They should be near the top of "Most Intelligent Dogs" lists, but these days, you hardly ever see them listed at all. It's a shame.