Lifelong United fan McIlroy told reporters on Wednesday how he approached Keane at a hotel as a kid and asked the former Red Devils midfielder to give him an autograph.
However, Keane turned the then-youngster down which McIlroy admitted had made him keenly aware of the need to try and give autographs to fans - demonstrated when the 28-year-old was seen handing a young boy a ball last week in a clip that subsequently went viral.
Speaking about why he had made the gesture at the British Masters, McIlroy said: "I went and asked him [Keane] for his autograph at the Portmarnock Links Hotel and he said 'no'.
“He should be one of my heroes because I was, and still am, a huge Man Utd fan. But I remember I went and asked him for his autograph at the Portmarnock Links Hotel [in Dublin] when he was with the Ireland squad and he just said no. And it just sort of stuck with me ever since.
"So that's why if a kid asks me for an autograph, I always try to do it.”
"I'm sure he's all right, but it sort of stuck with me ever since.
He added: "Every time I need a new one [ball] I look for a young kid in the crowd between holes to give it to them.
"I was surprised how that little moment last week became so big. But it just shows what such a small thing to me can mean to a kid, and that kid might be inspired to go and play."