The prop, who has 42 Leinster appearances already to his name and was capped by Ireland against the USA in June 2013, left Leinster for London Irish at the end of the 2012/2013 season and has played 27 times for the Aviva Premiership club.
The unavailability of a number of props due to injury or international commitments over the next few months has necessitated this move with Academy prop Ed Byrne being the latest player to pick up a significant injury.
Leinster Rugby head coach Matt O'Connor said, "Unfortunately Ed picked up a knock to his knee late in the Edinburgh game. We had to wait for the injury to settle but scans then confirmed that he had an ACL injury which will keep him sidelined for up to six months. This is a huge blow personally for Ed who has had a very positive start to the season and is a very popular member of the Leinster Rugby set up. He is due to undego surgery in Dublin tomorrow but knowing Ed, he will come back from this set back stronger than ever and it is good to see him back and in and around the environment this week already working hard on his rehab.
"The injury forced our hand but we are delighted to be able to call on a player the calibre of Jamie, who is a full Irish international but crucially knows the set up here, knows the players and what is expected of him at Leinster. We are also grateful to London Irish for facilitating this move and Jamie has already trained with us and is getting up to speed as we speak on our calls and patterns."
Jamie Hagan trained today with the Leinster Rugby squad in Donnybrook.
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