Major element compositional analyses, K–Ar ages, δD‰ and δ18O‰ values for 30 zoned and unzoned hornblendes from Dalradian amphibolites and metagabbros, mostly in north Connemara are reported. Although the cooling ages are expected from previous U–Pb zircon studies to be c. 475–450 Ma, the results obtained are from 556±6 to 410±9 Ma with an average of 470 Ma. Fluid movements, probably at 275±15°C, i.e. much below Ar closure temperature for hornblende, erratically reset the ages, as is shown by a negative correlation of hornblende δD and age and a wide scatter of ages even within 2 m. The changes were implemented by H–D exchange between fluid and hornblendes in which ionic porosity, Z, influenced the loss of Ar and possibly its gain from the fluid to give the excess Ar found in some samples. Z is controlled by hornblende chemical composition. High Mg, Si and Mg/Fe and low Fe, Al, Ti, Na and particularly low K, amphiboles giving low Z values retained Ar more firmly and gained Ar more readily than compositions which had higher Z values, which gave younger ages. These range down to c. 400 Ma, being the age of the intrusion of the Galway Granite suite that initiated the fluid circulation. The scatter of ages is a consequence of incomplete equilibration, mainly because of the slow H–D exchange rate below 350°C and partly because the fluid permeated erratically in different areas and down cracks of all kinds, promoting irregular Ar movement. The meteoric fluid circulated through Connemara, the Galway Granite and at least some of the contiguous Silurian sediments of the South Mayo trough. These overlying sediments may have contributed to the water circulated in north Connemara which was slightly less negative δD than in central Connemara. For hornblende K–Ar ages to be a reliable measure of times of uplift and cooling, they need to be demonstrated to be free from the influence of hot fluids by showing no correlation of age with δD.