Easterby, 36, retired from professional rugby at the end of the 2006/'07 season but he has featured in all 12 league fixtures for Otley this season, who lie in second place in English National League Division 2.
The former Llanelli and Rotherham star made 59 Leinster appearances between 2004-'07 (including 14 in the Heineken Cup) and will be eligible to compete in both the Magners League and Heineken Cup competitions.
Leinster coach Michael Cheika commented: "We are delighted that Guy has agreed to sign a short-term contract with Leinster. Guy has huge experience in the game and was a popular member of the squad up until his retirement and 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."
Guinness PRO12
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