Rates of ice stream retreat over decades can be determined from repeated satellite surveys and over millennia by palaeoenvironmental reconstructions. Centennial to millennial timescales are an important temporal gap in geological observations of value in process understanding and evaluating numerical models. We address this temporal gap by developing a 3 ka and 123 km retreat time series for the lateral margin of the Irish Sea Ice Stream (ISIS), a major outlet draining the last British Irish Ice Sheet. The Llŷn Peninsula (NW Wales, UK) contains numerous ice-marginal indicators from which we reconstruct a robust chronological framework of margin oscillations. The landscape documents the retreat of a former marine-terminating ice stream through a topographic constriction, across a reverse bed-slope and across variations in calving margin width. New dating constraints for this sequence are presented and integrated in a Bayesian sequence model to develop a high-resolution ice retreat chronology. Our results show that retreat of the ISIS during the period 24–20 ka displayed centennial-scale oscillatory behaviour of the margin despite relatively stable climatic, oceanic and sea level boundary conditions. Faster retreat rates coincided with greater axial trough depths as the ice passed over a reverse bed-slope and the calving margin widened (from 65 to 139 km). The geological observations presented here over a 123-km long ice retreat sequence confirm that marine-based ice can be inherently unstable when uncoupling from a reverse bed-slope, but also shows that the widening of the calving margin width can contribute towards destabilisation of the ice front.