The Irish offshore contains a large number of Late Palaeozoic to Cenozoic sedimentary basins, most of which are only lightly explored. These are grouped into the Irish Sea, Celtic Sea and Atlantic Margin basins. The basins developed in response to multi phase rifting which preceded the development of the North Atlantic Ocean. The major rift. phases occurred during the Permo-Triassic, Late Jurassic and Early Cretaceous. The Irish Sea basins contain a preserved succession of predominantly Permo-Triassic and Early Jurassic age, with younger strata largely absent due to Cretaceous and Tertiary inversion. The Celtic Sea basins have a thick series of Triassic to Cretaceous strata, overlain by a generally thin Tertiary succession. The Atlantic Margin basins have a variable sedimentary thickness, with the larger basins characterized by a thick Tertiary succession. Two gasfields are currently in production in the North Celtic Sea Basin, while a number of undeveloped oil and gas accumulations have been discovered in both the North Celtic Sea Basin, and the Porcupine Basin on the Atlantic margin. A wide range of reservoir and source-rock horizons have been encountered in the various basins. Most of the exploration to date has concentrated upon structural traps, but the recent resurgence of exploration interest is centred mostly upon a variety of stratigraphic traps, especially at Cretaceous and Tertiary levels, similar to successful plays further along strike in the UK and Norwegian sectors of the Atlantic Margin.