Exploration wells drilled offshore west and north-west of Ireland penetrated a c. 3.7 m cumulative thickness of Permo-Carboniferous sediments overlying Dalradian metasediments intruded by Caledonian granitic plutons. In the north, late Tournaisian to early Visean clastics were deposited coeval with the Rheic transgression upon an emergent craton in a continental to marginal marine environment. They record an increasing marine influence upwards with the establishment of a carbonate shelf in the Asbian. Clastic/carbonate sedimentation, which became cyclical in late Visean to early Namurian, temporally correlates with the Yoredale cyclothems. Younger Namurian sediments are not preserved. Sedimentation in early Westphalian progressed from prodelta to coal-enriched delta-top and abandonment facies, and is conformably succeeded by brackish to freshwater sediments that contain evidence for local marine incursions of Stephanian age. An Autuno-Silesian section, that is preserved offshore, may also have been deposited onshore prior to erosion. Seismic data enable partial delineation of N-S and NE-SW early sub-basins marginal to the Main Porcupine Basin. These sub-basins which contain Silesian sediments, may have styled successive extensional episodes in the basin and augment support for rift models of Carboniferous basin development in the North Atlantic/NW European domain. BIRPS data provide supporting evidence for the offshore prolonga­tion of pre-Mesozoic basement fault-zones.

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