The climatic effects of historical volcanic eruptions are well documented in the literature. What are less certain however, are the effects of eruptions on more distant environments, particularly vegetation. Here we present sub-annual δ13C records from two high-resolution Irish oak (Quercus spp.) chronologies that span the Laki (Grímsvötn) 1783–84 and Tambora 1815 eruptions. In both instances, a significant depletion in δ13C is recorded within the trees following the eruption (∼1.8‰). Historical meteorological datasets from observatories near to the trees sampled demonstrate that the shifts in carbon isotopic content cannot be accounted for by changes in local climate. We postulate atmospheric loading of ejecta from the eruptions resulted in significantly reduced irradiance, increasing discrimination within the trees.