A 1000 year long subdecadal-resolution record of carbonate oxygen isotopes ( delta O-18(c)) from Lough-na-Shade, Ireland, provides evidence for changing atmospheric circulation over northwest Europe. The total range of delta O-18(c) values (>5 parts per thousand) is too large to be explained by changes in water temperature. Moreover, good correlation between the lake record and a previously published delta O-18 time series from an Irish speleothem indicates that the changes in oxygen isotopes are best explained by variations in the isotopic composition of precipitation. The amplitude of change during this period is too large to be explained by shifts in condensation temperature. Instead we suggest that there have been changes in vapour source and transport paths connected with shifts in atmospheric circulation. Changes from a source area from further south within the North Atlantic to one further to the north could explain the prominent positive shift in oxygen-isotope values between the early eighteenth and early nineteenth centuries, for example. Our results also demonstrate the value of a 'multiple-archive' approach to deconvolving lake-based carbonate isotope profiles, which are often complex.