Some of the books and games deal lightly with the time period between the Treaty of Algeron and The Neutral Zone, but there isn't anything solid in the tv shows or movies that discuss it. We do have references to certain events, such as the massacre at Khitomer, the cloning of Picard, stealing B4 from Soong's lab, and so forth but that's about it. So there really isn't anything in canon which covers it. Probably the result of the writers strike interrupting the intended story arc that The Neutral Zone was supposed to start. That's probably a good thing anyway because, as I understand it, the original plan was to have the Borg completely wipe out the Romulan race after the events of that episode.
As presented in some of the RPG game modules by Last Unicorn, during much of the early part of the that time period the Romulans were expanding their empire in directions away from the Klingons and Federation, where they didn't have much resistance. As a result they came across a race known as the Taurhai Unity and war erupted. There was a video game that depicted a rogue Vulcan raising an army and attacking various Romulan bases and worlds as well. One or two of the novels also deal with a couple of coups that occured during that time, so civil war was also part of the flow of events. And as I recall, a couple of the comics also dealt with some events right around the time of their return.