The province have confirmed that the hooker has completed his time with Munster after securing a contract with a club in France which reports suggest may be Grenoble.
Casey has played 46 times for Munster and scored five tries since making his debut against Perpignan in the Heineken Cup at Thomond Park in December 2013.
The Cork man made 16 starts in 20 appearances that season and was named Munster Rugby Young Player of the Year.
The former Glenstal Abbey student played a part in the knock-out stages of last year’s title-winning British and Irish Cup campaign, and although the academy graduate has not featured for the senior side this season, he has lined out with his club side Shannon and Munster A.
Commenting on Casey’s departure, Munster Rugby Team Manager Niall O’Donovan said: “Duncan has been a massive character in our squad and will be sorely missed by this group of players and staff.
“He has worked hard in coming through the Munster pathway and has overcome a number of challenges in getting himself back to full fitness.
“We wish Duncan all the best with his next opportunity in France.”