So ridiculous, in fact, that England head coach Eddie Jones hinted at a conspiracy.
Was the late change to Ireland’s starting back row, named by his opposite number Joe Schmidt last Thursday and explained by a pre-match hamstring injury to No.8 Jamie Heaslip, pre-planned, he was asked?
“I don’t really care, mate. We’ve got to play against the 15 that’s out on the field,” replied the Australian, before then suggesting it was.
“If they want to do that, then that’s fair enough. Maybe a leprechaun tackled him in the warm-up.”
Schmidt begged to differ, of course, and was mildly affronted at the accusation that Heaslip’s withdrawal had been tactical rather enforced.
“That couldn’t be a more flawed theory,” the Ireland boss replied.
“It’s probably a bit of a slight on us. Because it’s not something we do. We pick our team and go out and play. It was a disruption if anything for us.
“Dan Leavy got a heck of a shock when I told him (he was on the replacements).”
Leavy was the final piece in the back-row rejig, taking O’Mahony’s place on the bench with the Munster captain installed as blindside flanker and his Munster team-mate CJ Stander moved from six to the No. 8 berth.
And boy did O’Mahony make an impact, marauding the Aviva Stadium pitch with snarling intent, making an utter nuisance of himself at the breakdown and restoring potency to an Irish lineout previously lacking in variety or threat.
The Cork man made telling contributions on both sides of the ball, leading the Irish tackle count with 13, none of them missed, and giving lineout caller Donnacha Ryan an added quality target for hooker Rory Best to aim at as Ireland launched the 23rd minute drive that ended with lock Iain Henderson stealing over for the only try of the game.
And crucially, in the final quarter as England chased the victory that would give them back-to-back Grand Slams and a record 19th consecutive Test victory it was O’Mahony who turned the tide, stealing a lineout replacement hooker Jamie George had meant for Maro Itoje in the 73rd minute with the game on a knife edge, the home side hanging precariously on to a 13-9 lead.
“Watching them play last week (against Wales), Paul O’Connell said he’s a massive lineout threat both in attack and defence and I guess he showed that today,” England No.8 Billy Vunipola said of O’Mahony.
“He’s got great athleticism, he’s great in mid-air... but we should have done better.”
For Schmidt, O’Mahony’s performance was exactly what he wanted.
“I said congrats (to him), that’s what we needed at the end of it,” the Ireland head coach said. “And then before I said ‘you know it, you are up for it, go and deliver it’.”
Yet the man himself said he was only doing his duty.
“I think Jamie took a bang in the warm-up and we went back in the dressing-room and I got the shout. It’s what we’re there to do, you prepare as if you are starting — not just the starters, but the subs. Dan Leavy came in as 23rd man and he did a great job coming on and making an impact. We’re happy to do it.
“It was a massive one for us. We needed to show everyone how much it meant to us and I think we did that today.
“It was a nice end to it but this team is big enough to be competing for championships and at the end of the day we want silverware.
“So we’ll take a lot of positives out of today and what went on over the last few weeks, and move on and build.”