The lavas of NE Ireland represent the largest remnant of the British and Irish Tertiary Igneous Province. In this paper we present new Sr isotope data which, by virtue of extreme variation, provide evidence of extensive open system behaviour in the petrogenesis of these flood basalts. To some extent the initial 87Sr/86Sr ratios (87Sr/86Sr(60Ma) = 0.703–0.715) record the effects of late-stage hydrothermal processes. However, leaching experiments allow primary magmatic signatures to be inferred with confidence. Coherent increases in initial 87Sr/86Sr ratio with differentiation indices and incompatible trace element ratios imply a role for crustal contamination. Very significant differences in the style of crustal interaction are recorded by discrete units within the basalt pile. Further, there are also marked contrasts with Tertiary basalts elsewhere in the United Kingdom. In both cases such variations appear to respond to changes in the basement geology reflecting the important control of crustal wall-rocks on the geochemistry of these basalts.