Sirius IX was an inhabited planet. It was the ninth planet in the Sirius planetary system. This system was located in the Alpha Quadrant.
Prior to stardate 4978, while on Sirius IX, Harry Mudd sold a love potion to a thousand people. When they became ill, Mudd quickly left the planet. (TAS: "Mudd's Passion"; Star Trek VI: The Undiscovered Country, production art)
Background
Star Chart
The Enterprise computer's star chart was a drawing by Rick Sternbach that had originally been published in 1979 on page 77. This chart showed Earth commercial and exploration routes after the use of warp drive began. Sirius A was a destination on an exploration route.
"The Explored Galaxy"
"The Explored Galaxy" was a map of charted space in the Alpha Quadrant. Sirius was located between the Tholian Assembly and Romulus. Both locations were identified in DS9: "The Search, Part II", "Call to Arms" as being located in this quadrant. This star chart was also seen in several Star Trek: The Next Generation and Star Trek: Deep Space Nine episodes set in the 24th century, from the year 2364 to 2370. These were: TNG: "Conspiracy", "The Measure Of A Man", "The Emissary", "The Mind's Eye", "The Game" and DS9: "In the Hands of the Prophets", "Cardassians".
Star Trek: Star Charts
In 2378, Sirius IX was a Federation member. This planet was located in the Beta Quadrant. (pgs. 45, "United Federation of Planets I")