"You can't say anything to that man."
Two years ago against Toulon, as Steffon Armitage seemingly pilfered almost every single loose ball that dropped to the deck, Sean O'Brien's patience snapped with the referee.
Trouble is, the same referee will be in the middle of the emotional, heaving cauldron that will be Cardiff's Principality Stadium this evening as Ireland attempt to keep their heads while 80,000 rabid supporters slowly lose theirs.
"He's not open to any kind of feedback," a still fuming O'Brien told us a few days later. "We've learned that in the past. So it was just one of those things where you get on with it and try and sort it out yourselves."
Wayne Barnes has a history with Ireland; two years ago at this venue, he also annoyed Ireland with many of his calls; and, were it not Gavin Henson's reluctance to kick, Ireland may have lost a Grand Slam here in 2009 thanks to a late Barnes call, even if his whistling of Paddy Wallace was, in fact, correct.
Whatever the rights and wrongs, Ireland's players cannot attempt to referee the game themselves. They must keep their cool in the broiling heat beneath the closed roof.
"Yeah, maybe, yeah," O'Brien concedes now. "I've probably let it get the better of me before. I've a job to do for the team and the ref has a job to do.
"We'll leave him do that and look after our own shop this week. Anything outside of the group isn't that important to us. It's about what we bring to the table and how we look after one another."
O'Brien's exasperation against Toulon and Wales were, he affirms, merely momentary lapses.
"I don't do it too often," he says. "I have done a few times in my career yeah but it's about channelling it in the right direction, that's the biggest thing - making sure I've a cool head and do my own job, looking after what I have to do rather than looking after what other people have to do or what the referee is doing."
Ireland have constructed their success on the plank of an exemplary disciplinary record.
"It's definitely a big thing that we've worked on, our discipline in high pressure situations, like this game will be," adds O'Brien.
"It's going to be a key factor in what way the game goes. It's something that we've been very good at, and we have to ensure that we stay good at it."
Joe Schmidt pointed out the difficulties with Nigel Owens against France, over the Welsh referee's interpretation of players losing their feet at ruck time.
Teams, though, are supposed to adapt.
"It's tough to change on the hoof," counters O'Brien. "Can you really say to yourselves 'we can get away with a bit more or a bit less'?
"I don't think you can do it that way, because the referee might see a different picture and he might change.
"That's where there are risks of giving away a stupid penalty in a bad place, so we control the controllables and hopefully make his job as easy as possible with our own actions.
"So while you adapt to different things in the game, you can't be touching the boundaries too much because it's so fast out there and if he sees a different picture, it's risky.
"We've just got to make sure discipline is top notch again."
O'Brien has enough to occupy himself in what should be an enthralling battle amongst the back-rowers; his own personal duel with Justin Tipuric has been an epic for some years now, culminating in a gripping Lions selection saga four years ago.
While much of the media attention was lavished on Warren Gatland's axing of Brian O'Driscoll, not to mention Jamie Heaslip, the Carlow man edged Tipuric in the race to replace the hamstrung Sam Warburton.
However, tonight is about lionhearts, not Lions.
"You don't think Lions," O'Brien says, assessing the Groundhog Day challenge of, well, the groundhogs.
"I've played against them a whole lot already and obviously we want to try and win that battle.
"It's going to be a tough day - for them and for us - around that area. You look forward to that challenge, too.
"Tipuric has always been a very consistent player, he's a very good footballer.
"Warburton is a class act, too. He's very hard to move when he gets over the ball. He's a good carrier as well, it's an element of his game he has improved a lot on."
The respect comes the other way, too.
"I got on really well with Sean on the Lions tour, Jamie Heaslip too," says Tipuric, destined to win his 50th cap for Wales this evening.
"They are both very good players. The trio mix it up quite well. They are known for their choke tackle. Sean and CJ Stander both like to get over the ball.
"CJ tries to choke you a bit and Sean is very over the ball and looks for the jackals. Same as Jamie as well, you can't forget him for what he's done for Ireland and he mixes it up as well.
"So you go to expect everything really, because they'll mix their game up and you've got to be switched on for everything they do."
Ireland's newly-configured back-row has shifted responsibilities; O'Brien admits he is doing "dirtier work" rather than enjoying bullocking gallops in the open prairies.
This thoroughbred hasn't become a mere workhorse; it's more of a subtle shift in emphasis.
"Sometimes you've got to try and win an inch, and an inch might lead to another inch a phase later," he explains.
"In an ideal world, yeah, you'd love to have a bit more space and be moving on to more ball at a rate of knots
"Having CJ and Jamie, there are more people carrying, so I'm letting them do a bit more work!
"My stats are still pretty high for carrying but so are theirs and that's good for us that we're sharing that workload.
"CJ had a massive amount of carries the last day and it's great that it's a collective thing.
"A few years ago I might have had double the carries of the next fella, but that's not the case now."
His body has changed, mostly through injury, but so has his temperament. And with greater wisdom comes greater strength.
Comment & Analysis
This may be our men's first Six Nations game on a Friday night, but Friday nights have been synonymous with Irish women's rugby for years and are something I have always loved.
Ireland will need more than clear eyes and full hearts under these Friday night lights. With the roof closed at the Millennium Stadium, Wales' backs are against the walls and the locals are demanding a performance.