Guy Easterby has come out of retirement to rejoin Leinster on a short-term contract until the end of the Heineken Cup pool stages.
Guy Easterby has played for English club Otley this season
The former Ireland international will bolster Leinster's scrum-half depth in the absence of injured Chris Whitaker.
Easterby, 36, quit professional rugby at the end of last season.
However, he has been in action for Otley in England's National League Two this season and has agreed to rejoin the Irish province.
Leinster coach Michael Cheika said: "Guy has huge experience in the game and was a popular member of the squad up until his retirement.
"I have no doubt he will give us the depth in the scrum-half position that we need at this important part of our year.
"He is a proven leader and it is also an added boost that he is familiar with the set-up.
"I would also like to express my thanks to Otley for their understanding and co-operation in facilitating the move."
Easterby will be eligible to play in both the Heineken Cup and the Magners League.
Chris Whitaker suffered his broken ankle in the Heineken Cup defeat by Toulouse.