In 2370, Barnaby was serving as a security officer on the USS Enterprise-D when he, along with other members of the crew, transported down to a rogue Borg planet in search of Lieutenant Commander Data who had unknowingly been captured his android brother Lore.
After searching the planet Barnaby was one of the last to return to the ship, where he relieved Ensign Taitt at the tactical station. He served in that position under the temporary command of Doctor Beverly Crusher due to many of the senior staff still being on the planet.
Due to the presence of a Rogue Borg ship in orbit, the Enterprise was forced to abandon the planet temporarily. Barnaby later implemented a plan to drop out of warp as close to the planet as possible in order to rescue the few remaining crew members of the search party, making the precise calculations needed to ensure success.
Initially, he was skeptical of Ensign Taitt's abilities due to her lack of experience, but she soon proved herself after coming up with an experimental plan to successfully destroy the Borg ship using a solar fusion eruption, thereby allowing them to safely rescue the crew from the planet. (TNG: "Descent, Part II")
"If your calculations are even slightly off, we'd hit the atmosphere."
"I'll have to be sure my calculations are accurate, Ensign."
"If her calculations are off, that eruption could encompass us."
"Well I'll just have to make sure my calculations are accurate, Lieutenant."
|Tactical officers of the starships Enterprise|
|Enterprise NX-01:||Alex • Billy • Reed • Unnamed • Greer|
|USS Enterprise:||Kelowitz • Lang • O'Herlihy • Chekov (post refit)|
|USS Enterprise-D:||Armstrong • Bailey • Barnaby • Burke • Hayes • Kellogg • McDowell • Rainer • Smith • Taitt • Williams • Worf • Yar • Unnamed • Crusher • Gaines|
|USS Enterprise-E:||Armstrong • Daniels • Kelly • Worf|
|ISS Enterprise NX-01:||Reed • Unnamed|
Barnaby was played by recurring Star Trek guest actor James Horan who also played the role of Jo'Bril in "Suspicions", a Takaran scientist who attempts to steal the very metaphasic shielding technology later used in "Descent, Part II", and in doing so endangers the life of Doctor Crusher. When Crusher discusses the technology with Barnaby in "Descent, Part II", he replies, "I know about that research."
During production the character was rumored to become a recurring one but ultimately this never materialized. 
The script for "Descent, Part II" describes Barnaby as, "an older, seasoned veteran." 
Barnaby appears as a selectable crewmember in the video game Star Trek: Starship Creator which gives his first name as "Albert" and describes him as serving with distinction throughout his Starfleet career. It also reveals that he was born in 2343 on the planet Zadar IV to mother Lee and father Edward, which would make him 27 years of age in "Descent, Part II". His biography also states that he has a brother named Harold who lives on Meezan IV. It goes on to explain that he graduated Starfleet Academy in 2364 in the 91st percentile with his first assignment being as a security officer aboard the USS Enteprise-D. In 2366, he was promoted to lieutenant junior grade and two years later went on to be selected for tactical field training in which he excelled during simulations. Following this, he was promoted to lieutenant and assigned as relief tactical officer on the Enterprise. After the destruction of the ship in 2371 he was posted to the USS Enterprise-E in the same position, serving with distinction during the Borg encounter at the Battle of Sector 001 in 2373. His biography lists his interests as volleyball, wind surfing and martial arts, in which he has a third degreee black belt in Tae Kwon Do.
According to his StarTrek.com biography, Barnaby's first name is Barthalomew.