During the Neutral Zone Incursion of 2266, the location of Romii was labeled on a star chart 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.
This location's quadrant of origin is inferred based on the location of Sigma Nelvana's system, in the vicinity of outposts 3 to 6, as seen in the star charts appearing in the Star Trek: The Next Generation episode "The Defector" and in the Star Trek: Picard episode "Maps and Legends".
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"), Romii was the name of a star system. The primary of the Romii system was a K-class star. It was located in the Romulan War Theater of Operations (2156-60). 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 system.