In southwest Ireland 2,500 m of Upper Famennian to basal Namurian marine sandstones and mudstones, the Cork Beds, overlie rocks of Old Red Sandstone facies. Coastal exposures of the Cork Beds are interpreted as showing gradual upward change from alluvial strata, through thick subtidal and shelf sediments to pyritic muds. A review of recent palaeontological evidence shows that the thick shallow marine part of the Cork Beds is older than the major development of limestones north of the Cork Harbour-Kenmare line, whose equivalents to the south are in the condensed basinal sediments. The Lower Carboniferous portion of the Cork Facies is shown to be thicker in South Cork than in West Cork. In Lower Carboniferous times a positive area-the Glandore High-separated two subbasins with different depositional histories. Six palaeogeographic maps are used to demonstrate the progressive shift of facie belts as Lower Carboniferous marine transgression progressed. Finally, brief comparison is made with rocks of the same age in southwest England.