A petrographic analysis of Cambro-Ordovician sand- and silt-sized greywackes from south-east Ireland indicates that much of the matrix is secondary in origin. Detrital quartz, feldspar and rock fragments are observed to contribute to the matrix; the quartz by marginal corrosion and accompanying recrystallization, and the feldspar and rock fragments principally by sericitization. The coarser greywackes show an upward decrease in matrix content through the succession corresponding to an increase in detrital quartz, feldspar and rock fragment contents, and to a decrease in the quartz content of the matrix. Most of these trends, albeit more ill-defined, also occur in the invariably more matrix-rich fine-grained greywackes. In the light of the available evidence, these trends and the petrographic features of the greywackes are best explained as being due to diagenesis, influenced primarily by the substantial overburdening effects of the sediment pile. A subsequent phase of lowgrade regional metamorphism is thought to have had little effect on the diagenetic features.