Fluid inclusion studies of granite quartz provide an opportunity to study fluid flow associated with igneous activity and post-emplacement fluid processes. This study presents new fluid inclusion data from the late Caledonian Donegal granites and Newry granodiorite, and the Tertiary Mourne Mountains granite in Ireland, which identify three distinct fluids. Aqueous-carbonic fluids (Type 1) have been recorded in late Caledonian granites with a significant mantle component (Newry granodiorite and the Ardara and Thorr granites in Donegal). These fluids represent late-magmatic fluids trapped at high temperatures (up to 575°C), and the ultimate source of these carbonic fluids is linked to sub-lithospheric processes during the Caledonian orogeny. The dominant fluid type (Type 2) in late Caledonian granites is a H2O+NaCl±KCl fluid which may be related to thermal convection cells around granite bodies and/or to regional scale influx of surface derived fluids at the end of the Caledonian orogeny. High salinity NaCl–CaCl2 fluids (Type 3) overprint quartz in the Ardara granite in Donegal, and in the Newry granodiorite, and are interpreted to represent basinal brines, sourced in overlying sedimentary basins, which circulated through the crystalline basement during a period of crustal extension (possibly during the Carboniferous or the Triassic). Fluid inclusion studies of the Tertiary Mourne Mountains granites have identified only Type 2 fluids related to thermal convection cells, consistent with stable isotope evidence, which indicates that this younger granite is unaffected by regional-scale fluid influxes.