The Romulan system was an inhabited star system in Sector Z-6. The system consisted of a primary and four planets. The second and third planets were Romulus and Remus, the homeworlds of the Romulans and Remans, respectively. (TOS: "Balance of Terror", display graphic; Star Trek VI: The Undiscovered Country, production art; DS9: "The Search, Part II"; Star Trek Nemesis)
During the Neutral Zone Incursion of 2266, the location of the Romulan system was labeled on a map of Sector Z-6 that was displayed on the bridge viewscreen of the USS Enterprise. (TOS: "Balance of Terror", production art)
In 2293, the location of Romulus and Remus in the Milky Way Galaxy was labeled in a star chart that was in Captain James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)
According to Startrek.com, the primary star in the system was called Romulus. According to Star Trek Maps (page 29) and The Worlds of the Federation (page 148), the Romulan star system was a binary star system of two white dwarfs known by the Federation as Romulus (primary) and Romulus B (secondary). In Diane Duane's Rihannsu novels, the Romulan system is a single star, known as 128 Trianguli to the Federation and Eisn to the Romulans ("homesun" in the Romulan language).
According to the Star Trek: Star Charts (pg. 66), this system was located in or near the Romulan War Theater of Operations (2156-2160).