The Irish Sea ice stream was the largest ice stream to drain the last British ice sheet, and around the margins of the Celtic Sea it deposited a shelly diamict facies that has been variously interpreted as subglacial till or distal glacial marine mud. We present new observations on the sedimentology of the shelly diamict and overlying sediments from the south coast of Ireland, and demonstrate that the shelly diamict facies is a subglacial deformation till deposited during onshore movement of a grounded Irish Sea ice stream. Stratigraphic and chronological data indicate that this occurred during the last glaciation, and this is regionally consistent with marine geological evidence from the Celtic Sea. We propose that the deforming bed tills along the south coast of Ireland are the onshore record of a rapid advance of an unstable Irish Sea ice stream into the Celtic Sea. This advance was facilitated, at least in part, by a saturated substrate of readily deformable, fine-grained, marine sediment. This implies an unstable and dynamic Irish Sea ice stream during oxygen isotope stage 2, and therefore extension of the last British ice sheet into the Celtic Sea far south of its traditionally interpreted limits.