He has a total of 135 appearances for Leinster and 38 caps for Ireland. The 34 year old Irish forward will team up with another Sarries newcomer, Andy Farrell, the former Rugby League star who has switched codes to Union.
Bernard Jackman, recently signed by Leinster from Connacht, is now expected to be in the front line for Leinster.
Saracens will face 1999 Heineken Cup champions Ulster, Biarritz Olympique and Benetton Treviso in Pool 4 of the Heineken Cup. Leinster will meet Bath, Bourgoin and Glasgow Rugby in Pool 5.