The Sinjoh Ruins are a place accessible on HG/SS. You can obtain a Level 1 Giratina, Dialga or Palkia here if you have an event Arceus in your party. You can reach it via the Ruins of Alph.
They are called the Sinjoh Ruins, as you said, as they are actually part of Johto and are home to the Sinnoh legendaries. Take the two words and you've got your name; Sinjoh Ruins.