The 29-year-old lock forward joined Tigers in the summer of 2005 and has since made 52 appearances for the club. He will return to the province from which he joined Tigers after the conclusion of the 2006/07 campaign.
"Leaving Leicester was a difficult decision, and I had to think long and hard about it," said Cullen. "I have really enjoyed my time with the Tigers and I feel that I'm a better player for the experience. The training environment is second to none and it has been great to have been part of such a prestigious club.
"We are still well placed in all three competitions and it would cap off two great seasons personally if I could help play a part in winning some much sought after silverware. I'd like to thank all my Leicester colleagues, especially my fellow players, and all the coaching, management, fitness and medical teams.
"What I will remember most about my time with Leicester is the fans - they are just outstanding the way they get behind the team. You couldn't ask for better supporters, and I will always think very highly of them. Playing at Welford Road is a special experience and to lead the team out that I have admired for so many years on a few occasions was a real privilege.
"I am sorry to be saying goodbye to Leicester Tigers but when I got the opportunity to come home to Leinster, I knew the time had come to do so. I am a Leinsterman born and bred, my family and friends are all there, and that is where I want to be."