Facies modelling of the internal structure of drumlins has resulted in recognition of five major facies associations within the Irish drumlin belt. The facies associations are linked with a sequence of depositional events during the last deglacial phase (ca. 20-16 ka BP). Drumlin sediments were formed by basal till deposition, subglacial fluvial-lacustrine sedimentation, sheet flow, debris flow and subglacial cavity deposition. Subglacial deposition was initiated by the infilling and blocking of subglacial networks which caused a reduction in hydraulic transmissibility at the base of the ice sheet prior to drumlinisation. Drumlins then formed as subglacial erosional forms during surge-type events when subglacial hydraulic transportational processes were active and sediment fluxes to the ice margins were high. It is argued on geologic evidence that basal boundary conditions of this type coupled with high rates of ice wastage by calving at the ice margin contributed to rapid disintegration of the last ice sheet in western Britain.