A gravity low immediately E of Dublin is interpreted as an Upper Palaeozoic to Cenozoic basin on the basis of seismic reflection profiles and direct sampling evidence. Maximum thickness of the sediment fill may exceed 4 km. It is considered to consist predominantlyof Carboniferous, undivided Permo-Triassic, and remnant proved Liassic. Thin Tertiary units, which unconformably overlie the main fill, are linked to sediments of similar character found in the Central Irish Sea Basin. Thick Quaternary deposits complete the sedimentary sequence. The basin is structurally a half graben located between two narrow, fault-bounded basement ridges, the Mid Irish Sea Uplift to the S and the Lambay Ridge to the N. The whole structure is bisected by a major NNW-SSE trending fault zone which with the margin faults has created two trapdoor-like structures. It is thought that the principal phases of movement which were instrumental in preserving the sediments occurred during the Triassic and late Jurassic period.