A lithic assemblage is described from numerous localities on the shores of Lough Allen, part of the River Shannon system in Ireland. By macroscopic and microscopic comparison with locally collected rock, the vast majority of the 869 artefacts are determined to be worked from a type of silicified dolomite, a finegrained rock type with nearly isotropic fracture properties not previously recognised amongst Irish prehistoric lithic assemblages. The geological distribution of this type of silicified dolomite is restricted to Lough Allen and the upland and lowland areas west, north and east of the lake shore. It is demonstrated through typo-technological analysis that the vast majority of the artefacts are from the Later Mesolithic, forming the second largest Mesolithic surface collection, after Lough Gara, in the west of Ireland. The collection includes a small number of axe roughouts, the first to be recorded in a lakeside setting in Ireland. The analysis provides firm evidence for extensive Later Mesolithic activity in the Lough Allen area based mostly on locally available material. 14C dating of an in situ oak trunk from the lake shore is also presented and indicates lake levels lower than those of today during the Bronze Age, which together with the distribution of lithics suggests that a broader lake shore may have been exposed during the Later Mesolithic.