“This is one of the hardest decisions that I have ever had to make. I have played all my rugby in Ireland,” said O'Brien. “That being said, I feel that the time has come to explore other opportunities, to challenge myself at a new environment, in a new city, with a new club and against different players and teams on a weekly basis.

“London Irish, and what they are about, is a club that I feel I can grow with and also contribute to and I feel that they have a set up and an ethos that will make me feel at home.”

O'Brien has won four league medals and four European titles with Leinster. He has also won Six Nations titles with Ireland but the move, when it happens, will bring an end to his international career given the IRFU's unwritten decree that prioritises the use of players playing for the provinces.

“Seán has been a key driver of Leinster Rugby’s success over the last ten years and played a pivotal role in our four Heineken Cup-winning seasons,” said Leinster head coach Leo Cullen who played alongside him prior to his own retirement.

“He has not only been a world-class player on the field, but has also made an incredible impact in growing the game all over the province. He has also been a model professional and a shining example to our young and Academy players.

“As a teammate, Seán was the sort of competitor you always wanted beside you. He is the kind of player who gives you the confidence that anything is possible – nothing fazes him and he always led from the front.

“Everyone at Leinster wishes only the very best for Seánie in his future as he embarks on a new challenge. We will all miss him greatly, but his legacy here will live on as he has influenced so many of us in such a positive way.

“For now, we hope to finish this season strongly and, after his disappointment of missing out on a number of big games last season through injury, we hope Seán can play a major part in Leinster’s push for silverware in two competitions this season.”

The 'Exiles' are currently nine points clear at the top of the IPA Greene King Championship, England's second tier, having been relegated from the Gallagher Premiership last season and O'Brien's capture is testament to their ambition.

Former Munster and Ireland head coach Declan Kidney is Director of Rugby at the club and ex-Ireland defence coach Les Kiss is also a member of the management setup. O'Brien played under both, earning 27 of his 54 Ireland caps under Kidney.

“Recruitment is an ongoing process, and we are always on the lookout for players of Seán's ability,” said Kidney. “ Seán has proven himself time and time again to be a quality player who sets very high standards for himself and those around him, he leads by example and has a winning mentality.

“We are delighted that Seán wants to come and be a part of our journey as we move back into West London at the new Brentford Community Stadium in August 2020 after he has finished his commitments to Leinster and Ireland at the end of this year.”

O’Brien said of his soon-to-be new club: “After speaking with Declan it was obvious the ambition London Irish have as a club. I’m keen to roll up the sleeves and use my experience to help re-establish London Irish as a top club in England.

“It’s a club that’s been around for a long time and there’s an obvious connection to Ireland with some greats of Irish rugby having previously worn the jersey. For now my concentration remains solely with Leinster and Ireland but it’s an exciting challenge that awaits.”