Romulus in the mirror universe was, like Romulus in the "primary" universe, the Romulan home planet.

Star Trek: New EmpireEdit

In Star Trek: New Empire, Romulus was quite different compared to the primary Romulus. Romulans were more like Vulcans of the primary universe, whereas mirror-Vulcans were generally more like primary Romulans. The mirror-Romulans challenged the Terran Empire, resulting in an attack which stripped Romulus of its atmosphere, rendering the planet's surface barren, forcing inhabitants to live in domed cities or underground.

It's been hinted that the Terran Empire may have been only indirectly responsible for this attack, as apparently it occurred around the mid-24th century, decades after the Terran Empire fell.

Romulus became the seat of the Romulan Technocracy by the late 24th century. Despite an avowed political neutrality, Romulus surreptitiously aided the Terran rebellion against the Klingon-Cardassian Alliance. By 2385, this "unofficial" policy had reversed, with Romulans seeking to aid the Alliance, then later Klingon guerrillas against Starfleet forces of the Imperial Union of Planets. A Starfleet invasion in late 2385, following the ISS Arcadia's destruction near Romulus, forced Romulus to surrender. By early 2386, Romulus was fully in the grip of Imperial occupation and had joined the IUP. Imperial delegates met in the Romulan capital city, where a statue of Stephen April had been erected.

The Romulan Technocracy employed Remans, who were commonly seen on Romulus. (Star Trek: New Empire)