Inch Abbey is the most atmospheric ruin near Downpatrick. The Cistercian abbey was founded in the 12th century on what had once been a drumlin island in marshy ground, and its remains still sit beautifully beside the River Quoile.
The ruins are open and free, with enough surviving stonework to make the abbey plan clear without turning the visit into a lesson. Come for the setting as much as the masonry, especially if the light is good over the river.