Shadowmoon Ruins was a location listed on the map of Hellfire Peninsula in The Frozen Throne campaign. Little is known about it, and it wasn't visited during the campaign. It is unclear why Shadowmoon would have had any landholdings in Hellfire Peninsula.
Perhaps the ruins were intended to be the ruins of Fortress Shadowmoon, although it was located in a different location than in Warcraft II's Shadowmoon lands (Shadowmoon Valley). Alternatively Shadowmoon Ruins could be intended to just random ruins of previously unknown Shadowmoon landholdings in Hellfire Peninsula...or they may be ruins of what was once part of the southern Peninsula, now known as Shadowmoon Valley, that had somehow crashed into Hellfire Peninsula during the world's destruction.
What the TFT designers had originally intended is unclear.