He will link up with his colleagues from that Ireland squad, Andrew Browne, Thomas Anderson, Eoghan Grace and Niall O'Connor, in the green of Connacht next season.
The 5ft 10in, 89kg scrum half has made 16 appearances to date for Leinster, including two run-outs in the Heineken Cup against the Scarlets.
O'Donohoe also started for Leinster in Connacht's New Year's Day defeat at the RDS earlier this season.
The former Belvedere College pupil, who turns 24 in August, has captained Leinster 'A' in recent season. He has played 19 times for the province's 'A' side, including eight times in the British & Irish Cup.
Commenting on the new deal, the Dubliner said: "I am delighted to be signing for Connacht next season. I worked under Eric (Elwood) and Dan (McFarland) in my Ireland Under-20 days.
"I have first hand experience of how passionate they are about Connacht and also how ambitious they are, so I am very excited by this challenge.
"I will obviously miss the lads and the backroom staff at Leinster as I have been here since my Academy days, however Connacht is a new chapter in my rugby career that I am very excited about."
Connacht head coach Eric Elwood added: "We're delighted Paul is joining us. I've known Paul since he starred for the Ireland Under-20s in 2007.
"I believe he's a top class player and very capable of the standard required in the Magners League and European competitions. He's a young dynamic scrum half and will fit very nicely into our set-up here."