Sirius IX was the inhabited ninth planet in the Sirius system, approximately eight light years from Sol. (TNG: "The Naked Now" production art) The planet had at least a thousand inhabitants, who allegedly had an unique biochemistry.

At some point before stardate 4978, Harry Mudd sold a love potion to a thousand people on Sirius IX. When they became ill, Mudd quickly left the planet. (TAS: "Mudd's Passion")

This planet was only mentioned in dialogue.
According to Star Trek: Star Charts (pp. 45, 60, & 66; "United Federation of Planets I") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Sirius (Alpha Canis Majoris) system was located in the Beta Quadrant. In 2378, Sirius IX was listed as a Federation member.
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