The RIS Sharelth is a fairly new warbird, commissioned in 2378, and has some of the newest weapons and shield technology available. It was also a part of the project designed to help fight the Borg off during their attempts to invade Romulan space. When the Romulan Empire, under Praetor D'Varin, agreed to become a part of the Allied effort against the Kiklar, Asgalad was promoted to Commander and was given command of the Sharelth, which was then assigned to the Sentinel's battle group.
I know even less about the Romulans than I do about the Klingon ships. The RIS (Romulan Imperial Starship) tag for their ships was just a guess. All I really know about Romulan warbirds is that they are able to cloak, use disruptors, and are generally pretty large (though the Valdore in Nemesis seems to show that these new warbirds are smaller than the massive D'Deridex warbirds.) In the series, I named them Valdore-class warbirds since the only named ship I knew of was the Valdore. I've since learned that the class is called Norexan. I'd rather not have to, but it looks like I'll have to go through the episodes involving these ships and make sure I change the class name...