The Monac shipyards were Cardassian-Dominion shipyards located above Monac IV, near the Monac star.
In 2375, a Klingon fleet attacked the shipyards but failed to destroy them, suffering losses of over thirty percent in the process. Shortly after, General Martok and Commander Worf successfully destroyed the shipyards using just the IKS Rotarran, by triggering a solar plasma ejection from a magnetic instability on the Monac sun. They had undertaken the mission to ensure Jadzia Dax's entry into Sto-vo-kor. (DS9: "Image in the Sand", "Shadows and Symbols")
The Monac shipyards were designed by John Eaves and named after DS9's special effects supervisor Gary Monak. (Star Trek: Deep Space Nine Companion, p. 598)
According to the Star Trek: Star Charts, on page 46, the Monac shipyards were located in the Alpha Quadrant.