Cronin, 24, was educated at Ardscoil Rís in Limerick and has previously played with Munster and Connacht, whom he joined in the summer of 2008.
A former Ireland Schools representative, Cronin featured in the AIB League for Shannon and he earned his first Ireland cap against Fiji in the RDS in November 2009, incidentally in the same match as Jonathan Sexton and Sean O’Brien.
He won his first start against New Zealand in the 2010 Summer Tests and also started against Australia. Cronin has also represented Ireland Schools, U19, U21, and ‘A’ (Ireland Wolfhounds) levels.
The Limerick man played three games in the recent Guinness series against Samoa, New Zealand and Argentina and has featured 7 times for Ireland to date.
Sean Cronin said: “I am delighted to put pen to paper on a deal to join Leinster this summer and I believe that with the exciting players in the squad, the future looks bright.
“I have been hugely impressed by Leinster’s displays both in the Magners League and in the Heineken Cup so far this season and the province’s ambitions mirror my own. Hopefully I can establish myself in the squad when I join up in a few months time.
“I am grateful for the opportunity that Connacht have given me over the last three years to play for a fantastic club. They will continue to grow from strength to strength under Eric Elwood and the existing management team and I will do my utmost to help improve our Magners League standing over the coming months.”
Commenting on Cronin’s signing, Leinster Coach Joe Schmidt said: “It’s great that Sean has confirmed that he will join us this summer. He is a dynamic and talented front rower who will add experience at hooker following the retirement of John Fogarty earlier this season.”