It also led to fellow professionals dubbing him the next Tiger Woods after he lifted his first Major.
McIlroy, 22, came to prominence in Northern Ireland at an early age.
In July 2005, shortly after turning 16, he went round Royal Portrush on the region's dramatic North Coast in a course record. He became European amateur champion the following year - it qualified him for The Open - and reached number one on the world amateur rankings.
He was third at the US Open last year and third in the last two US PGA Championships. But one of his biggest disappointments was at the US Masters in Augusta this year when he was the youngest player ever to lead the competition but his game collapsed on the final day.
McIlroy's masterclass was reminiscent of his boyhood idol Tiger Woods' domination of the 2000 US Open when he won by a record 15 shots at Pebble Beach.
McDowell said: "He's potentially the next Tiger Woods - he's that good. It's great to see him out there fulfilling his potential. He's been like an accident waiting to happen."