New isotopic ages are presented from the syntectonic Cashel-Lough Wheelaun basic intrusion and an associated K-feldspar granite gneiss of the Connemara gneiss complex from Lettershinna Hill. The linear, normally discordant distribution of data-points from the Cashel-Lough Wheelaun metagabbro indicates that the zircons in this intrusion have no significant inherited radiogenic Pb. They yield an upper concordia intercept age of 477+25−6 Ma, and a more precise 207Pb/206Pb age of 490 ± 1 Ma. In contrast, zircons from the K-feldspar granite gneiss show distinct reverse discordance. The non-magnetic fractions show a good linear arrangement yielding a lower intercept age of 454+6−14Ma and a very uncertain upper intercept age of 1807+646−570Ma. The lower intercept age is younger than the 477 ± 9 Ma K-Ar age of hornblendes in gneisses from Lettershinna Hill (Elias et al. 1988) and therefore reflects a Pb loss episode younger than igneous crystallization. The imprecise Proterozoic upper intercept age indicates contamination by older inherited and only partially reset zircons.