In Colonel J.K. Milner’s The Irish Setter: Its History & Training (1924), the author describes black and sable Irish setters, as well as the more well-known solid red and red and white dogs.
What I find interesting is a statement that appears at page 46:
The late Mr. Cecil Moore told me he had some excellent black Irish setters. The black setters are the result crossing with black dogs…Some black retrievers in Ireland are said to be descended from red setters.
My guess is the black dogs the author is writing about are solid black Welsh setters and maybe black collies and black and tan dogs that eventually became the Gordon setter breed.
We do know that the wavy-coated landrace of retriever had a lot of setter in it. It also had St. John’s water dog, collie, and a little water spaniel in it.
Black dogs, of course, are the dominant color to the red or yellow dogs, so if a black Irish setter had been bred into the retriever lines, it could have passed on the recessive color into the retriever bloodlines. Dogs that probably descend from black Irish setters carrying the red color are “Breeze” and, of course, “Nous.”
Now, I do not discount that there could be other sources for this color, including cream colored Featherstone castle setters, true red collie dogs (not sables or livers), and, of course, various water spaniels and water dogs, including the Tweed water dog.
However, I think that looking at the setters tells a lot about how this red to yellow color was introduced into retrievers.