This report is a contribution to the Strategic Environmental Assessment (SEA6) conducted by the Department of Trade and Industry (now Department of Energy and Climate Change). This report discusses the seabed geology and the surficial geology and processes of the SEA6 region of the Irish Sea. The East Irish Sea Basin is at a mature exploration phase, having produced oil and predominantly gas since 1985. The hydrocarbon geology of the region is reviewed. Other prospectivity plays in the SEA6 area are the Caernarfon Bay Basin and the Cardigan Bay Basin. The seabed sediments, seabed sedimentary processes and the corresponding diverse range of seabed habitats in the area are described, drawing upon published data and on the new information acquired from surveys specifically conducted for this SEA in 2004.