The performance is one that Dunne can be optimistic about, a win is what he wants but falling short to a golfing prodigy and beating him for three out of four days is impressive.
It's the 23 year old's fifth professional title and third European title, but it's the first time he has won as a professional on Spanish soil.
His biggest challenge on the day came from another Spaniard, Nacho Elvira, who made back-to-back birdies at 13 and 14 to get within one of the world No. 4.
South African Gerry Coetzee surged 31 places through the field to finish fourth with a best round of the day 63.
The final pair both bogeyed the fourth to briefly hand Rahm the advantage, only for Dunne to nudge in from close range at the par-five next and Elvira to drain a 35-foot eagle to grab the lead.
"It's been unbelievable. 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".
His win will probably see him move ahead of Jordan Spieth in the golf world rankings to number three and begin to threaten Justin Thomas and Dustin Johnson at the top of the world rankings.
"I've been blessed to be national champion from 16 to all ages in Spain". There, he escaped with a par after nearly sending his tee shot into the water, the ball just holding up in the rough. As a golfer it's hard to deal with the public's emotions, so I tried to isolate myself and that's perhaps why I didn't play my best golf. "So being able to win this as a pro and do this for them and the Spanish people feels great".
That left Dunne or even Elvira needing an albatross on the 18th to induce a tie an order with Dunne having a birdie into pip the Spaniard to location.
Spaniard Jorge Campillo and Scot Marc Warren carded closing rounds of 69 to finish at 15 under alongside Australian Brett Rumford and Swedish rookie Henric Sturehed, who picked up his first European Tour cheque.