Ryn III was a planet in or near Federation space. It was the homeworld of the Ryns.
In the mid 2330s, Ryn III was a major distribution hub for "red ice" being dealt by the Asfar Qatala crime syndicate, as well as a base of operations for the rival Orion Syndicate. During this period, Starfleet Intelligence came to suspect Orion operations on Ryn III were facilitating weapons trade between the Breen Confederacy and the Cardassian Union.
In late 2335 or early 2336, a team of three Starfleet officers from the USS Barker -- Commander Samir al-Halak, and Lieutenants Strong and Thex -- were assigned a covert mission to Ryn III, posing as traders willing to travel across the Cardassian border. They were quickly discovered (after being betrayed to the Orion Syndicate by Strong) and were forced to flee the planet. (TLE novel: Well of Souls)