The Romulan Republic is the mirror universe counter part of the prime universe's Romulan Star Empire. Like the prime Universe, these Romulans also engage in the Earth-Romulan war of the mid-22nd century with the Terran Empire, after which the Romulans constructed series of defense space stations to defend their space from future Terran attacks. Contact between the Terran Empire and the Romulan Republic had been opened by the 23rd century.
In 2269, the USS Eagle crossed into the Romulan Neutral Zone while being chased by the Terran ship the ISS Eagle. Seeing the markings were different from the other Terran ships of their universe, they opened fire on the Terran vessel and offered assistance to the Federation ship and helped them return to their own universe.
Romulans of this universe are considered ruthless but also chivalrous towards their enemies but hospitable and helpful toward their friends and allies. (Star Trek: Eagle)