Dunne, who sprang to prominence after sharing the 54-hole lead in the 2015 Open Championship whereas nonetheless an beginner, fired seven birdies and an eagle in a closing 61 to complete 20 below par.
And the 24-year-outdated sealed victory in type by chipping in on the 18th to complete three photographs away from McIlroy, whose 63 was the bottom rating of an harm-plagued season which he'll deliver to an early, self-imposed finish subsequent week.
Ryder Cup vice-captain Robert Karlsson, who started the day with a one-shot lead, completed third on 16 below with Graeme Storm, David Lingmerth and Florian Fritsch on 14 below.
Dunne, who misplaced a play-off to Edoardo Molinari in the Hassan Trophy in April, loved a outstanding stroke of luck when his strategy to the 11th pitched right into a sprinkler head on the sting of the inexperienced and bounced again to inside 5 ft of the opening.
However the luck was properly deserved after the 24-year-outdated performed the first six holes in 5 below par to maneuver right into a lead he would by no means relinquish, with the victory - value €566okay - lifting him into the world's prime 100 for the first time.
McIlroy was at all times taking part in catch-up regardless of a hat-trick of birdies from the sixth and his likelihood of a first win of the 12 months regarded to have gone when he missed good probabilities on the ninth and 10th and bogeyed the following.
Nonetheless, the 4-time main winner refused to throw in the towel and carded 4 birdies in 5 holes from the 12th, earlier than coming inside inches of driving the inexperienced on the brief par-4 17th.
From a difficult lie on the sting of a greenside bunker, McIlroy chipped to 4 ft and holed for birdie to shut inside a single shot, however a prolonged birdie try on the final was by no means on line.
That left Dunne needing to par the ultimate two holes for victory however he birdied the 17th and chipped in for one more on the final, a becoming finish to per week which noticed document crowds of 60,180 in attendance.