As Rahm closed with a five-under 67 to win his third European Tour title on 20-under par and become only the sixth Spanish victor since 1972, Dunne had to console himself with a late birdie that denied Spain's Nacho Elvira a share of second at the Centro Nacional de Golf.
He said 'I felt like I hit the ball great. Some of them skidded and some of the bounced off. I was hitting it long and straight. "It was just one of those days, but my game is great and I feel good going in to the next few weeks". "My short game let me down a little bit".
Dunne bounced back on the next to stay in contention but Elvira found the water for a double-bogey on the 17th and with Rahm birdieing the last, the title was sealed. "It's truly been the hardest Sunday I've ever had in any tournament that I've won because the crowd wanted it so much and I wanted it so much".
In addition to his stellar driving, Rahm carded 21 birdies and two eagles set up by his superior iron and wedge play. "Spain has a great champion in Jon and I'm sure he'll win many more", said the Irishman. It was really hard to deal with the public's emotions. Disappointed not to win but Spain has a great champion in @JonRahmpga. World No. 4 Jon Rahm (66) and Henric Sturehed of Sweden (66) finished the day two strokes back.More news: Trump warns Russian Federation on Syria: 'missiles 'will be coming'
"It's such a satisfying feeling", the world number four told europeantour.com.
"I'm just happy to play the way I did and to get it done", Rahm said after his victory.
Further down the leaderboard, South Africa's George Coetzee would finish solo fourth on 16 under, four off the pace, while a host of players, including Marc Warren local Jorge Campillo, were locked for T5 on 15 under.More news: Constand Denies Defense Claim She Plotted to Extort Cosby
Elvira also birdied the par five first and there was very quickly a three-way tie for the lead at 17 under.
Rahm did not look likely to take advantage of the par five tenth after sending his second right and his third over the green but he chipped in and led on his own, with Dunne hitting back on the same hole when he got up and down from a bunker.
Rahm would only birdie the 13th and 18th.
There was a two-shot turnaround when he bogeyed 11 and Rahm holed a curling birdie putt on 13, as Rahm moved to 19-under par.More news: Teenager unearths treasures linked to Harald Bluetooth
While he birdied the 15th to get back to within two shots of Rahm and Elvira, there were to be no late mistakes from the eventual victor.