This ancient stone wall is located in the Mourne Mountains of County Down, Northern Ireland. Built in the late 19th century, the Mourne Wall is a dry-stone wall that runs for 22 miles across the mountain range. The wall was built to protect the land from sheep and cattle
From Wikipedia
The Mourne Wall was constructed to enclose a catchment area of the Silent Valley Reservoir in the Mourne Mountains, Northern Ireland. The 1.5 m high stone wall, which was built to keep livestock from contaminating water supplies, took almost twenty years to complete. The project was overseen by the Belfast City and District Water Commissioners.