The British Geological Survey was commissioned by the Crown Estate to assess geological conditions in the Irish Sea in relation to the possible constraints they may place on development of offshore infrastructure. The report describes the geology between 0 m and 50 m below seabed, which is the depth most relevant to current pile foundation technology. The report reviews the best available data from a variety of sources including, BGS legacy data, map sheets, and regional reports, as well as site investigations carried out for hydrocarbon and offshore renewable industries. Additional data collected for the proposed Round 3 offshore wind farm in the Irish Sea (Celtic Array) was also included. Prof. Richard Chiverrell and Dr. Katrien Van Landeghem (of Liverpool and Bangor universities), who together have extensive experience working in the Irish Sea, provided valuable advice and guidance. The report is split into four principle sections as summarised below. Section 3 summarises seabed topography, sediments and processes. The topography of the report area is split into shallow platforms and deeper troughs. Seabed sediments are subdivided into regions of soft mud- (clay and silt) rich sediment in the eastern and western Irish Sea and a central gravel belt comprising coarse sand and gravel. Small areas of bedrock outcrop at seabed are also recognised. Currents in the Irish Sea mobilise sediment to form a collection of marine bedforms ranging from ripples to very large (up to 36 m in height) solitary sediment waves and banner banks. Predicting bedform migration speeds and pathways is difficult and requires repeat surveys. Bedform migration rates of 0 m/yr to 66 m/yr, with average values around 6 m/yr have been observed. Shallow gas is expected in some areas of the Irish Sea. Where this gas is present, pockmarks or methane derived authigenic carbonate may occur. Section 4 summarises the Quaternary history of the Irish Sea and its impact on the distribution, thickness and properties of sediment. Growth and collapse of ice sheets and associated sea level fluctuations principally determine geological properties of Quaternary sediments. The stratigraphy in the report area reflects three major glacial periods with the last one having the most pronounced influence. Very stiff diamicts (glacial ‘boulder clays’ or tills) are present across most of the report area of variable thickness. In enclosed deeps, locally sediment thickness can be >100 m. Glacial landforms are preserved at the seabed and can be used to predict sediment properties. Extensive studies onshore can provide analogues to assess potential geological properties offshore. Section 5 provides a review of bedrock distribution and properties. Where Quaternary sediment cover is <50 m, bedrock will be encountered in the shallow subsurface. The predominant bedrock lithologies in the report area are Triassic and Carboniferous sandstone and mudstone. Geotechnical properties of Triassic rocks are comparable and potentially predictable. Carboniferous rock show high lateral and vertical variability. There are a number of igneous intrusions in the report area and rock properties near to the location of these igneous bodies may differ due to alteration of the host rock during intrusion. Section 6 summarises the geological constraints identified in preceding sections with reference to engineering activities and infrastructure. The report outlines the current state of knowledge of geological conditions in the Irish Sea. It is recommended for use as a guide and should not replace a detailed site investigation.