Fault kinematics in Cardigan Bay during the Cretaceous/Tertiary are problematic. In spite of abundant evidence of basin inversion across the region, there are few recognized examples of typical inversion-type structural geometry. Sub-surface structural mapping of the southern part of the Cardigan Bay basin reveals evidence of sinistral oblique-reactivation of an Early Palaeozoic fault during the Tertiary. A new kinematic scheme depicts Cardigan Bay as one part of a structurally compartmentalized Irish Sea–Cardigan Bay–Celtic Sea basin system. In this model, pre-existing NE-trending faults in Cardigan Bay exhibit sinistral displacement within a larger, NW-trending dextral shear couple. The geometry and magnitude of Tertiary inversion within this basin system was largely dependent on the relative orientation of the Early Tertiary palaeostress to the basement lineaments within each sub-compartment. Some of the implications of strike-slip fault reactivation for hydocarbon exploration and production are discussed.