Leinster and Ireland flanker Sean O'Brien will join London
Irish when his central contract ends after the 2019 World Cup.
O'Brien has agreed a three-year deal worth around €420,000 per annum (£370,000) with the Exiles, who are currently nine points clear at the top of the Championship.
The back-row forward, 31, has earned 54 Ireland caps and was selected for the 2013 and 2017 British & Irish Lions tours, making five Test appearances.
?? London Irish can confirm the signing of Sean O'Brien, who will join the club in December 2019. Read more ?? https://t.co/ja7NSoJ4eV
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— London Irish (@LiRFC) February 11, 2019
On joining, O'Brien said: 'This is one of the hardest decisions that I have ever had to make. I have played all my rugby in Ireland.
'With Tullow, with Leinster and with Ireland and to move from the place I call home and that I love so much, was not easy.
'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.'
Ireland had opted not to renew the flanker's central contract due to concerns ove his age and recent fitness record.
O'Brien added: '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.
'For me and the time I have left here in Ireland, I want to stay fit and healthy and finish out the season with a strong Six Nations with Ireland and then turn my attention to Leinster and hopefully a seat on the plane to Japan.'
London Irish coach Declan Kidney added: 'Recruitment is an on-going process, and we are always on the lookout for players of Sean's ability. 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.'