Bidding to become the first wire-to-wire victor since Neil Coles in 1973, the Greystones star (25) had to make a 10-footer for birdie at 18th just to finish two shots behind the conquering home hero. "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".
"It's hard to describe the pride I have right now to be Spanish national champion having been a Spanish amateur champion many times with the Spanish Golf Federation, and to be able to win as a pro and do this for them and for the Spanish people, it feels great and hard to describe", he said to Sky's Andrew Coltart.
Dunne's second-place finish landed him a big pay day though, as the 25-year-old picked up a cheque amounting to €166k for his efforts. There, he escaped with a par after nearly sending his tee shot into the water, the ball just holding up in the rough.
Two ahead playing the par-five 18th, Rahm reduced it to a drive, a mid-iron and two putts on greens that were far from flawless. "I saw the willingness and the feeling everybody had".
After Rahm had stamped his mark on proceedings with birdies at the first, second and seventh holes in the final round, the tournament was decided on the short 17th. It was really hard to deal with the public's emotions.
Rahm (23) showed he was a special talent when winning two Rolex Series events a year ago - the Dubai Duty Free Irish Open and the season-ending DP World Tour Championship in Dubai. "When I made the decision to come straight from Augusta, it wouldn't be to just show up and walk around; I wanted to win this tournament".
Then for a fourth day in succession, Dunne birdied the par-5 fifth with his eagle chip shot from just off the green missing the cup by millimetres.
While he was tied for the lead when he birdied the 10th, the putts that fell for the first three days failed to drop.
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.
With his girlfriend, parents and grandmother in attendance, as well as the backing of a vociferous home support, Rahm certainly felt the pressure but turned it on when it mattered most just a week after recording his best major finish of fourth at the Masters last week.