Bajoran wormhole schematic
The ring singularity was a location in the Bajoran wormhole. This singularity was located between the negative CTL region and B3.
On the chart "A Tunnel in the Sky", which was seen in the schoolroom aboard Deep Space 9 in 2369, the ring singularity was one of the components labeled on a map of the wormhole. (DS9: "In the Hands of the Prophets")