The data set comprises measurement of physical and biogeochemical oceanographic parameters and complementary meteorology collected during the Liverpool Bay/Irish Sea Coastal Observatory initiative. It includes measurements from across the Liverpool Bay and Irish Sea area with data collection spanning a decade from 2001 to 2011. It incorporates regular hydrographic survey cruises (typically 8 - 10 per year) undertaken by the RV Prince Madog, data collected via instrumented ferries, time series data from oceanographic moorings and at two meteorological stations, namely: Bidston Observatory (up to 2004) and Hilbre Island (2004 - 2011), and a shore-based high-frequency (HF) radar measuring waves and surface currents out to a range of 50km. The hydrographic surveys include conductivity-temperature-depth (CTD) casts with attached auxiliary sensors and data collected via the ships' underway monitoring system. Oceanographic parameters include temperature, salinity, dissolved oxygen, attenuance, turbidity, fluorescence, chlorophyll, nutrients, irradiance, waves and currents and meteorological parameters include air temperature, air pressure, wind velocities, humidity, precipitation and atmospheric irradiance. The instrumented ferries also incorporated an underway monitoring system for sea surface properties. The Observatory integrated (near) real-time measurements with the POLCOMS (Proudman Oceanographic Laboratory Coastal Ocean Modelling System) models. The objective was to understand a coastal sea's response both to natural forcing and to the consequences of human activity. The foci were the impact of storms, variations in river discharge (especially the Mersey), seasonality, and blooms in Liverpool Bay. The Observatory was coordinated at the National Oceanography Centre (previously the Proudman Oceanographic Laboratory, POL) in Liverpool and data are managed by British Oceanographic Data Centre (BODC). The data set is supplemented by infra-red (for sea surface temperature) and visible (for chlorophyll and suspended sediment) satellite data. These data are held at the NERC Earth Observation Data Centre /Remote Sensing Data Analysis Service (NEODC/RSDAS).