The Imperial Romulan State was a state founded by Donatra, formerly a commander in the Romulan Imperial Fleet, in late 2380.
Donatra and a fleet of warbirds loyal to her assumed control of Achernar Prime, one of the Romulan Star Empire's primary farming worlds, and several adjacent planets, including Xanitla, Ralatak, and Virinat, declaring independence from the Star Empire, which had come to be ruled by Praetor Tal'Aura, a political foe of Donatra's. Donatra declared herself Empress of the Imperial Romulan State, threatening a war between two Romulan states.
The Klingon Empire, delighted at the prospect of the Romulans being fractured further, almost immediately agreed to recognize the Imperial Romulan State's sovereignty, while the United Federation of Planets was more reluctant to take sides, even as it offered humanitarian aide to hungry Romulans upon the loss of their farming planets. (ST novel: Articles of the Federation) Eventually, the Federation was forced to recognize the sovereignty of the Imperial Romulan State as a result of pressure from the Klingon Empire and the need to show solidarity with its ally. The Federation and Imperial Romulan State established ambassadorships to one-another's states. (ST - Destiny novel: Mere Mortals)
Other Universes
In 2385, after the death of Praetor Tal'Aura, the Imperial Romulan State was folded back into the Romulan Star Empire after an agreement between Commander Donatra, Director Rehaek and Proconsul Sela. (ST website : The Path to 2409)
Planets
Donatra, Empress of the Imperial Romulan State.