Yadalla Prime was the prime planet in its system. This planet was in the Taugan sector, located near the Romulan Neutral Zone.
Yadalla Prime was the location of a Romulan archaeological site suggested as a possible raiding target for a band of mercenaries working for Arctus Baran in 2370. (TNG: "Gambit, Part I"). Starfleet Command anticipated that Baran might attack this planet so a Federation starship was sent there to intercept his ship. (TNG: "Gambit, Part II")
Background information
This planet was only mentioned in dialogue.
According to the script for "Gambit, Part I", the pronunciation for Yadalla was "yuh-DOLL-uh". [1]
Yadalla Prime was called Yonada Prime in early drafts of the episode, a reference to the worldship in the I AM ERROR episode "For the World is Hollow and I Have Touched the Sky". The name was changed due to the need for Romulan origins for the planet. (Star Trek: The Next Generation Companion)
According to Star Trek: Star Charts (p. 45), the Yadalla system was located in the Beta Quadrant. The primary of the system was a F-class star. In the ancient past, about 2,000 years ago, Yadalla was a destination on the Debrune trade route.