This dataset contains raw water temperature, conductivity (derived salinity) and pressure data from CTD instruments from a pilot deep water mooring deployed at a site (15.52 degrees West, 52.999 degrees North) in the South Rockall Trough close to the M6 met-ocean buoy. The pilot sub-surface mooring was deployed in October 2018, then retrieved in May 2019 and consisted of an array of 10 Sea-Bird SBE 37 CTD sensors at a series of fixed depths below the surface (500m, 625m, 750m, 1000m, 1250m, 1500m, 1750m, 2000m, 2500m, 3000m). This subsurface mooring pilot deployment collected data providing a short intra-annual time-series (~8 months) and complements the annual Marine Institute oceanographic survey to the South Rockall Trough, which has taken place since 2004 weather conditions permitting. The CTD sensors were fully calibrated by Sea-Bird in Germany both pre and post deployment. Additional sensors on the mooring include ADCP sensors to measure ocean currents and direction. The mooring was built, deployed and managed by the Marine Institute through discretionary Marine Institute funding and loan of sensors from the National University of Ireland Galway (NUIG). At present there are no operational or project funds available to maintain a continuous series of moorings at the South Rockall Trough location but when logistics and infrastructure are available future deployments may take place.