The SmartBay Observatory in Galway Bay is an important contribution by Ireland to the growing global network of real-time data capture systems deployed within the ocean. Installed on the seafloor 1.5km off the coast of Spiddal, the observatory uses cameras, probes and sensors to permit continuous and remote live underwater monitoring. Data relating to the marine environment at the site is transferred in real-time from the SmartBay Observatory through a fibre optic telecommunications cable to the Marine Institute headquarters and then made publicly available on the internet. This dataset comprises of processed data that has been collected from the SmartBay Observatory site using a Seabird SBE 16 Conductivity-Temperature-Depth (CTD) sensor (formerly using an Idronaut Ocean-Seven 304 plus sensor – see history below). The sensor is located on the Observatory at a depth of 25m. The sensor measures the temperature, conductivity, pressure and oxygen concentration of the seawater. Salinity, oxygen saturation and sound velocity are calculated from these measurements and included in the dataset. This dataset comprises of the processed CSV data files containing these measurements. Practical uses of this dataset include but are not limited to scientists, researchers and marine technologists involved in the area of Physical Oceanography. This is a continuously updating dataset and includes data collected using a now retired sensor. The timeline of sensor use is as follows: Oct 2015 – Nov 2018 - Idronaut Ocean-Seven 304 plus Oct 2021 – present - Seabird SBE 16