“It's been incredible,” he said on Monday of his time at Leinster. “There's been a few tears shed over the last few months thinking about all of this. When you do make your final decision, it's a tough place.

“At the end of it all you're just packing your bags and you're walking out the door and moving to a different club. It hasn't been that easy but it is what it is. You back yourself to go over there and do a job."

That job will amount to a three-year contract wit the Exiles and recent evidence suggests that it will all but end his Ireland career given the manner in which Ian Madigan, Donnacha Ryan and Simon Zebo have all been frozen out.

Picking players who are under the auspices of the IRFU's player welfare programme may be an unwritten rule but it is strictly enforced and O'Brien will be 32 years of age by the time he makes the switch to his new abode in the English capital.

He claims he has yet to talk to Andy Farrell about his situation post-Japan. The Englishman takes over from Joe Schmidt at that point and, at the beginning of a long four-year World Cup cycle, it is all but inevitable that the Ireland squad will undergo some change.

That said, O'Brien remains gung-ho about his Test prospects come 2020. There is no sense that he is approaching this Six Nations as one last valedictory tour through the old championship before he closes the book on it for good.

“I haven't actually because if I'm fit and well and I think I can add value to this group. Even after I leave Leinster, I'd hope to be selected.

But, again, that's so long away and it's probably a decision that the coaches and whoever is in charge at the time will have to make. But there's always hope there, that regardless of where you are, you're still in with a shout if you're playing well enough.

O'Brien is understandably eager to turn the spotlight on what is happening in the here and now with Leinster, who are looking to defend their PRO14 and Champions Cup titles, and Ireland in this World Cup year.

But there is an excitement about the prospect of beginning a new chapter on his career with a club that is expected to confirm its promotion back to the Gallagher Premiership in the not-too-distant future under Kidney, under whom he made his debut with Ireland a decade ago.

“He wants to bring the club to a real competitive place in the Premiership hopefully, develop a lot of the younger guys and create a really good culture there. He’s done a great job from what he’s said to me and what I have seen so far in bits and pieces.

“Obviously it’ll be a different kettle of fish when they get back into the Premiership. He didn’t have to sell it too much to me, they are things I enjoy doing. There are a lot of younger lads coming through their academy, there is a big focus on them for the past couple of seasons.

“You can see a lot of them are getting their chances in the last few games. I have been keeping one eye on (them) as such. That’s a major part of it and the brand of rugby they are trying to play is going to be an exciting brand of rugby hopefully. They have some nice classy players.”