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.

The Silent Valley Reservoir is a reservoir located in the Mourne Mountains near Kilkeel, County Down in Northern Ireland. It supplies most of the water for County Down, surrounding counties and most of Belfast. It is owned and maintained by Northern Ireland Water Limited. The reservoir was built between 1923 and 1933 by a workforce of over 1,000 men, nine of whom died during construction.