Sedimentary basins west of Ireland contain a number of deep seismic structures that have been variously interpreted as fault blocks, serpentinite extrusions or igneous complexes. The Porcupine Arch (PA) is a deep-level (>11 km) domal 50 km wide seismic feature associated with a prominent free-air gravity anomaly high and high P-wave velocities. Detailed seismic mapping of igneous sill complexes in the Porcupine Basin suggests a possible connection with the PA. The sills form a thick (>5 km) interconnected network extending from the PA into the flanking post-rift Cretaceous stratigraphy, suggesting that the PA may be the top of a large (ultra)mafic intrusion that fed the sills. An intrusive origin for the PA is in agreement with geophysical modelling (gravity and Vp), the seismic character of the Porcupine Arch structure and the primitive bulk composition of the Porcupine sills, and is consistent with documented regional Cenozoic uplift in the basin with the development of shallow water and the occurrence of Paleocene–Eocene deltaic depositional systems. Similar mafic–ultramafic intrusive complexes have been documented elsewhere on the northeastern Atlantic margin, including the Rockall Trough. These findings emphasize that higher heat flow in the early Cenozoic may have prevailed over the northern part of the Porcupine Basin.