The Porcupine Seabight is a broad, composite rectilinear embayment in the margin of the Irish continental shelf which seems to be floored at least in the southern part by oceanic crust. There is a geometrically straightforward relationship between areas of crustal thinning, rifting, and the position of probable 'oceanic crust'. It is proposed that an important crustal unit here termed the Porcupine Segment includes the Porcupine Bank and the western end of the Slyne Ridge. Interpretation of a new magnetic map indicates that opening in the Porcupine Seabight appears to have taken place along a set of transform faults which 'root' in an older dextral shear fault set oriented at about 130°. Three major zones along the longitudinal axis of the Seabight relate to the geometry of magnetic stripes in the subjacent basement and can be used, in conjunction with opening related to a distant pole of rotation, to explain the overall present physical configuration of the basin. The Porcupine Seabight probably opened no earlier than about 170 Ma, following the establishment of a Jurassic volcanic province in the vicinity of the southern Seabight elbow.