Irish flanker O'Brien will have to accept referee Barnes is not for turning
Wales v Ireland (live rte2/bbc ko 8.0)
"You can't say anything to that man, there's no point. He's not open to any kind of feedback. We've learned that from the past."
This was Sean O'Brien's verdict on Wayne Barnes after Leinster went out of The Champions Cup to Toulon in Marseille 2015. The conciliatory tone towards referee Barnes adopted by Ireland this week was taken out of necessity.
It doesn't necessarily mean anything has changed. The same impression stands as the Irish provinces and Ireland have not had good memories of Barnes' manner or his matches.
For all of that, Ireland were incensed at the way Barnes took a zero tolerance approach to Irish players falling on the wrong side in the first quarter at Cardiff two years ago and completely changed his interpretation when Wales fell the wrong side of the ruck, but not the whistle, later in the game.
"I've probably let it get the better of me before," said O'Brien this week.
Job to do
"I suppose I've a job to do for the team and he (Barnes) 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."
It would be very easy for O'Brien to let the emotion of the occasion seep into his outward expression of frustration that often comes with the appointment of Barnes.
"It's about channeling it in the right direction.
"That's the biggest thing, making sure I've a cool head, looking after what I have to do rather than looking after what other people have to do or what the referee is doing."
Ireland take the best discipline, in terms of the concession of penalties, and the best scrum into Cardiff.
They will have to be squeaky clean in their actions and their technique.
"It's definitely a big thing that we've worked on, our discipline in high pressure situations.
"While you adapt to different things in the game, you can't be touching the boundaries (around the breakdown) too much," said O'Brien.
"It's so fast out there and if he sees a different picture, it's risky."
Once the English referee has made up his mind, the man is not for turning.