He was known as "Garth of Izar" during the 2260s. While in a fit of madness and upon hearing James T. Kirk's address of him as "Garth" or "Captain Garth", he corrected Kirk, calling himself instead, "Lord Garth, formerly of Izar." (TOS: "Whom Gods Destroy")
According to Star Trek: Star Charts (pp. 32 & 36) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the M-class planet Izar was the third planet in the Izar (Epsilon Boötis) star system, located in the Alpha Quadrant. The primary of this system was a K-class star.
According to Star Trek: Star Charts (pp. 32, 36), in 2183, after the first manned landing by the SS Horizon, an Earth colony was founded on this planet. Within months of being founded, the Izar colony was admitted to the United Federation of Planets. The dominant species were Humans. In 2370, the census counted 185 million living on this planet. The planetary capital was New Seattle. Major attractions on this planet included the Starfleet Tactical School and the Izar Institute of Meteorology. This world was a destination on a major space lane in the late 24th century.