The Shadowmoon Ruins is a location seen on the Outland map in Warcraft III: The Frozen Throne. Little is known about it, and it wasn't visited in the game. It is unclear whether the name "Shadowmoon" refers to the zone or the orcish clan (although they are related).
There are no locations in Outland in World of Warcraft carrying this location's name. Moreover, it's difficult to estimate where it, in any case, would have been located, due to the multiple errors related to the Warcraft III Outland map (see Hellfire Peninsula for more info). 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 be just random ruins of the previously unknown Shadowmoon clan landholdings in Hellfire Peninsula. They may also 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. In the TBC "Behind the Scenes" DVD, one of the designers stated that players had only visited Hellfire Peninsula during Warcraft III. However, as the Shadowmoon Ruins were not actually visited, it's still possible they are part of another zone.
What the TFT designers had originally intended is unclear.