Understanding the triggers and pace of marine-based ice sheet decay is critical for constraining the future mass loss and dynamic behaviour of marine-based sectors of the large polar ice sheets in Greenland and Antarctica. Numerical models which seek to predict this behaviour need to be calibrated against data from both contemporary and palaeo-ice sheets, and the latter requires accurate reconstruction of former ice sheet extent, dynamics and timing. Marine geophysics, sediment cores, benthic foraminiferal assemblages and radiocarbon dating are used to reconstruct the extent of the last British-Irish Ice Sheet (BIIS), and the timing and style of its retreat on the Atlantic shelf northwest of Ireland. Shelf edge moraines and subglacial till recovered in cores from the outer continental shelf are dated to younger than 26.3 ka cal BP and indicate an extensive ice sheet at the Last Glacial Maximum (LGM) that was grounded to the shelf edge. Nested arcuate moraines record the subsequent episodic retreat of the ice sheet across the shelf. Lithofacies and associated foraminiferal assemblages demonstrate that this retreat occurred in a glacimarine environment as a grounded tidewater margin and that high relative sea level and cold waters prevailed during retreat. Radiocarbon dates indicate that the timing of initial ice sheet retreat from the shelf edge occurred in the interval between 26.3 and 24.8 ka cal BP, during the period of minimum global eustatic sea level, and that the ice sheet had retreated to the mid-shelf by 24.8 ka cal BP. The ‘Donegal Bay Moraine’, a large moraine at the mouth of Donegal Bay, records a major stillstand and readvance of the ice sheet during deglaciation between 20.2 and 17.9 ka cal BP. Estimated retreat rates of 5.5-35 m a-1 across the shelf demonstrate that retreat was slow. It is noteworthy that retreat was initiated in the absence of ocean warming and when eustatic sea level was at a minimum. The sea-level rise that initiated deglaciation from the shelf edge therefore, is inferred to have been a product of local glacio-isostatic crustal depression rather than external forcing. This demonstrates that marine-based sectors of ice sheets can trigger their own demise internally through glacio-isostatic adjustment and it provides an explanation for the early retreat of the BIIS on the Atlantic shelf during the global LGM (gLGM).