Abstract: Until recently, there have been relatively few natural examples of basement fault reactivation, demonstrating convincing hard-linkage to underlying structure and a well-constrained three-dimensional (3D) geometry and timing of reactivation. Applying various quantitative techniques to high quality 3D seismic data, we describe the Cenozoic structure and growth of a reactivated Mesozoic normal fault in the NW Porcupine Basin, west of Ireland. We demonstrate a previously unrecognized period of Mid–Late Eocene extensional reactivation with a tectonic, rather than the previously suggested compactional, origin. East–west extensional reactivation of a north–south- to NE–SW-trending basement fault led to the development of a highly segmented and systematically stepping fault array in the Cenozoic cover. A clear relationship between the displacement of hanging-wall antithetic faults and displacement along the main synthetic system is attributed to the antithetics primarily accommodating strain associated with a hanging-wall rollover anticline. Our study shows that displacement analysis provides an excellent basis for defining the kinematics of basement fault reactivation, and the importance of fault segmentation and twisting during the upwards propagation of reactivated faults. Fault timing corresponds to a period of rapid differential subsidence, the magnitude of which is too large to be attributable solely to extensional reactivation of basement faults.