Accelerator mass spectrometer C-14 dating of in situ monospecific marine microfaunas and reworked shells records high relative sea level on the western coast of Ireland between ca.40 and 19 call (calibrated) ka, requiring substantial isostatic loading by the Irish ice sheet (IIS) during that interval. Glacial and marine deposits also record a rapid fluctuation of the US margin onto the continental shelf ca.28 cal ka. Evidence that the coast was not subsequently overridden by the IIS indicates that at least for this sector of the ice sheet, the ice margin during the Last Glacial Maximum (LGM) was less extensive than the 28 cal ka event. Our data from western Ireland support C-14 and Cl-36 chronologies from the Irish Sea Basin in identifying an US that was as extensive and thick as the LGM IIS for much of the 20 k.y. interval prior to the LGM.