Midway through his final round, the Spaniard was involved in a rules scare that threatened, briefly, to bring his irresistible momentum at Portstewart Golf Club to a screeching halt.
Rahm was approached by the European Tour’s chief referee, Andy McFee, as he walked off the 13th hole to discuss how he had marked his ball on the sixth green, a sequence of events that immediately brought back memories of the extraordinary four-shot penalty meted out to Lexi Thompson at the LPGA’s first major of the year.
Playing in the final group alongside American Daniel Im, Rahm originally marked his ball to the side, and then moved his marker one putter head away so his marker wasn’t on Im’s line. However, when Rahm replaced the ball, TV footage – and still shots – showed that he replaced the ball in front of the marker.
Not by a lot, maybe a centimetre, but far enough to be a clear breach.
In April, Thompson’s bearings at the ANA Inspiration were also out by a minute amount – perhaps two centimetres when she replaced her ball – but that didn’t stop the rules makers docking her four shots, two for the infringement and another two for a wrongly marked scorecard, effectively costing her the title.
Yet, after hearing Rahm’s version of events on Sunday afternoon, McFee decided the former Arizona State player had not intentionally replaced his ball incorrectly.
And the reason he gave – in a nutshell, that Rahm had made every effort to replace his ball in the right spot and what’s a few millimetres here or there, anyway? – will have many golf officials scratching their heads in puzzlement.
By accepting Rahm’s word that he had not intended to mark the ball closer to the hole, McFee has seemingly opened a can of worms: do tournament officials now believe every player who gives them a sob story about how they didn’t mean to break a rule? Or just some of them?
“We’re talking fine margins here,” McFee explained. “The reason why there’s no penalty is because I think Jon’s made a reasonable judgement. He’s marked the ball at say 10 o’clock. When he’s put the ball back down, he’s definitely not back down in front of the ball marker which would be 12 o’clock. We’re talking about the difference between the ball being lifted at 10 o’clock on the ball marker and put back at 11 o’clock, which is not a problem.
“Secondly, we’re talking about a player moving his ball marker off to the side to get out of the way of his fellow competitor and then moving it back again. Now there’s always going to be a small margin of error in this.
“I don’t think the ball was put down in exactly the right place, but I think it falls within the player’s made a reasonable judgement to put it back so that’s why there’s no penalty.
“One of the points in the new decision is that the outcome depends a lot on what the player says and his explanation of the events. Jon said: ‘I knew I marked it to the side and then I was trying to make an effort to put it back to the side.’ He’s definitely made the effort.
“The new decision the R&A and USGA crafted, with the full knowledge from the PGA Tour and ourselves, is all about trying to eliminate these fine margins and get to a position where if a player has made a reasonable judgement then the game will accept it if it’s slightly wrong.”
Golf Channel analyst Brandel Chamblee was one observer who felt McFee was overly lenient in the way he handled this situation: "Andy McFee certainly has a great reputation administering the rules in a fair manner, but I believe he got this one wrong. It wasn't millimetres. It was inches, probably two, three inches that this ball was misplaced. So, he [Rahm] broke the rule. He should have been penalised, which means he wouldn't have been playing with a five-shot lead. He would have been playing with a three-shot lead.
"All of a sudden, what looks to be something easy and a walk in the park becomes very stressful.”
No-one likes a rules Nazi, or a totally rigid, inflexible application of the rules when there might be mitigating circumstances, but in these two cases, it does seem as though what’s been good for the goose has not been good for the gander.
Thompson would rightly feel miffed that her error drew such a harsh sanction – especially given that it occurred during the previous day’s play, and was only brought to light by an eagle-eyed TV viewer – yet Rahm’s mistake was not penalised at all.
In the end, just to pile agony on top of her acute sense of injustice, Thompson lost in a playoff to South Korea’s So Yeon Ryu.
No such misfortune for Rahm, 22, who coasted home by six shots to claim the US$1.67 million Irish Open winner’s cheque, barge his way into the world’s top 10 – and become one of the warm favourites for the British Open at Royal Birkdale next week.
How different it could all have been if he was confronted by a less genial and accommodating referee than Mr McFee.