It is now generally accepted that British Tertiary granites contain crustal and mantle components. Genesis principally by differentiation of crustally contaminated basaltic magmas is widely held and silicic melts with some remarkable trace element similarities were generated within different upper crust along the St Kilda/Skye – Carlingford zone. New whole-rock (and mineral) O isotope data for the southern sector of the province (N Arran, Ailsa Craig, Mourne Mountains, Slieve Gullion, etc) reveal that δ 18 O lies in the range +5.1 to +9.7‰ for most of the analysed granites, meteoric water-rock interaction having been in general less intensive than at Skye and Mull. Nevertheless, highly 18 O-depleted country rocks (with δ 18 O<0) exist adjacent to the N Arran and Mourne Mountains granite plutons. There is as yet no evidence for the existence of low – 18 O granitic melts in this southern sector where magmatic δ 18 O compositions (up to c . +9.5‰) can be inferred for some of the intrusions. New Nd (and Sr) isotope data indicate that although there is some similarity in initial 87 Sr/ 86 Sr ratios between the northern and southern sector granites, in northeastern Ireland initial ε Nd values for the analysed Tertiary acid major intrusions range from −3.9 to −4.5. This is in marked contrast to the Skye granites, some of which have values below −20, reflecting the involvement of different lithosphere.