The U'lanna was a Beta Quadrant species long extinct. It possessed psionic technology allowing one to control the will of others. (Starfleet Command Volume II: Empires at War)
It was ruled by the U'lanna Commonwealth; the species was extinct 3,000 years ago because of a supernova. Also, the species was in the middle of a civil war at the time of extinction, with the U'lanna Rebel Alliance extant at that time. The race was notorious for shunning deflector shields, building mobile shipyards called astro-miners without shields. (Star Trek: The Stoneship Files: "A Knight without Armor")
The U'lanna didn't attack without provocation. (Star Trek: The Stoneship Files: "Generational Ghosts")
U'lanna-built starships remained within Lyran territory, mostly within the Iron Fang County. The U'lanna Campaign's ultimate goal was to destroy all U'lanna ships. (Star Trek: The Stoneship Files: "Ancient Horrors")
The U'lanna were brought back from extinction in 2402. They considered sexology, and medicine more generally, to be an important part of their lives, to the point they granted the first Doctor of Sexology degrees in Lyran history. (Star Trek: The Stoneship Files: "For a New World")