During the Neutral Zone Incursion of 2266, the location of Romii was labeled on a map of Sector Z-6 that was displayed on the bridge viewscreen of the USS Enterprise. (TOS: "Balance of Terror", display graphic)
This location was only mentioned in writing.
According to Star Trek Maps (p. 29), Romii was an early Romulan colony. It was a white subdwarf orbited by four planets, with only the inner planet inhabited, with a population of five billion.
According to Star Trek: Star Charts (p. 66, "United Federation of Planets II") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"; "Position of the Romulan Star Empire"), the Romii system was located in the Beta Quadrant. The primary was a K-class star. It was located in the Romulan War Theater of Operations (2156-2160). According to a map of the Romulan Empire – prepared by the Stellar Archive Artist Lsel Essik, from the Romulan Master Data Catalog, in 2366 – Romii was identified as a Romulan star.