The Leinster flanker’s last appearance at what was still known as the Millennium Stadium at the time was for Ireland’s 2015 World Cup game against the French when a swinging punch to the midriff of Pascal Pape was punished with a ban that saw him miss the quarter-final loss to Argentina.
Seven months earlier and the Carlow man was part of an Irish team that fell foul of referee Wayne Barnes’ strict interpretations on players rolling away from the tackle and he would vent his feelings about the Englishman in May of that year after Leinster fell just short of Toulon in a Champions Cup semi-final.
“You can’t say anything to that man, there’s no point,” he said of Barnes after the province’s defeat in Marseille when Steffon Armitage’s questionable poaching stance at the ruck got up his nose.
The criticisms of Barnes, who is referee again tonight, were more nuanced than that but that one quote stuck.
“I’ve probably let it get the better of me before,” O’Brien said this week of his emotions. “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. 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.”
Ireland conceded 13 penalties to 11 from Wales that day in March of 2015 but their discipline has been a talking point of late.
The last time Joe Schmidt’s men conceded ten or more penalties was the last June Test against South Africa, in Port Elizabeth.
The last time they conceded more penalties that the opposition was the week before, in Johannesburg.
Their numbers since then read: four, nine, four, three, seven and eight, which is suggestive of a team that knows how to read referees.
Still, this being rugby, it is far from a perfect science.
Barnes showed this time two years ago, when he relaxed his views on players rolling away after 20 minutes punishing Ireland, and just when Wales started to go on the defensive, that there is not always such a thing as consistency of application.
Most players talk about the need to recognise which way the wind is blowing and react accordingly but O’Brien all but dismissed the idea that players engaged in the combat of Test rugby can just flick a switch and do things differently.
“I don’t think you can do it that way, you see, 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.”
O’Brien’s own actions have changed.
Most of his duties in this Six Nations have amounted to what he calls “dirty work”. His carrying stats remain high, and he is happy with his form, but there have been fewer sightings of the barrelling Tullow Tank racking up the metres and making for open country.
He’s fine with that.
The presence alongside him in the back row of CJ Stander and Jamie Heaslip offer Ireland plenty of options when it comes to making headway although O’Brien isn’t of the belief that it is the arrival of the Munster captain into the neighbourhood that has curbed his flights.
“No, I don’t think there’s a different emphasis. If you look at the last day there was no line breaks or anything like that. It was more attritional against the French and you have to be as tidy as you can with your carries and small things, rucks and bits and pieces towards the maul, and keeping the scrum going.”
Whatever his duties, it’s a comfort to have O’Brien there. Tonight will mark the first time since the 2013 Six Nations, when he made all five games, that he will start four successive Test for Ireland after a succession of injury-hit seasons.
Little wonder he’s intent on keeping the head down and getting on.