The impending move had become public last week
London Irish have announced the signing of Leinster and Ireland flanker Sean O'Brien.
The move had been all but confirmed last week, before today's official announcement.
The 31-year-old will start life at London Irish in December 2019, after Ireland's Rugby World Cup campaign.
It will also see him reunite with ex-Ireland head coach Declan Kidney. The 2009 Grand Slam winning coach awarded him his first Ireland cap during that year's November tests.
Kidney is currently London Irish's director of rugby.
Sean 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 Sean 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," he told the official website.
O'Brien also highlighted Kidney's influence in helping him make the decision.
“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."
Speaking on Friday's Off The Ball, former Ireland captain Brian O'Driscoll says the move won't affect O'Brien's World Cup hopes.
"You have to remember Joe Schmidt is leaving after the World Cup too," he said.
"And, maybe if there was a situation where it was a 50/50 call between him and a Josh van Der Flier or a Dan Leavy for one reason or another, maybe then, it would work against him but ultimately because Joe is gone - he wants his best squad.
"He's not thinking about the future - he's thinking about his best players, his best leader - those who can win him a World Cup or get to a final of a World Cup."
✍️ London Irish can confirm the signing of Sean O'Brien, who will join the club in December 2019. Read more 👉 https://t.co/ja7NSoJ4eV
— London Irish (@LiRFC) February 11, 2019