Maureen O'Hara tells the story this way in her autobiography:
"There is only one fitting way to end our discussion of The Quiet Man, and that's with a whisper. No matter what part of the world I'm in, the question I am always asked is: "What did you whisper into John Wayne's ear at the end of The Quiet Man? It was John Ford's idea; it was the ending he wanted. I was told by Mr. Ford exactly what I was to say. At first I refused. I said, "No, I can't. I can't say that to Duke." But Mr. Ford wanted a very shocked reaction from Duke, and he said, "I'm telling you, you are to say it." I had no choice, and so I agreed, but with a catch: "I'll say it on one condition -- that it is never ever repeated or revealed to anyone." So we made a deal. After the scene was over, we told Duke about our agreement and the three of us made a pact. There are those who claim that they were told and know what I said. They don't and are lying. John Ford took it to his grave --so did Duke-- and the answer will die with me. Curiosity about the whisper has become a great part of the Quiet Man legend. I have no doubt that as long as the film endures, so will the speculation. The Quiet Man meant so much to John Ford, John Wayne, and myself. I know it was their favorite picture too. It bonded us as artists and friends in a way that happens only but once in a career. That little piece of The Quiet Man belongs to just us, and so I hope you'll understand as I answer:
I'll never tell"
The DVD featuring the audio comments you just heard, is on Amazon: The Quiet Man (Collector's Edition)