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 Edit
This planet was only mentioned in dialogue.
According to the script for "Gambit, Part I", the pronunciation for Yadalla was "yuh-DOLL-uh". 
Yadalla Prime was called Yonada Prime in early drafts of the episode, a reference to the worldship in the Star Trek: The Original Series 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.