Given the expansion of plantation forests over recent years there is a need to assess their impact on biodiversity in Ireland. Spiders make useful indicators of biodiversity for several reasons: they are primarily affected by vegetation structure which in turn is affected by habitat and disturbance; they occupy an important place in terrestrial food webs as both predators and prey; they are relatively immobile between habitats; and their ecology and taxonomy are well known. Pitfall traps were used to sample spiders in a range of forest types, of different species composition (Sitka spruce, ash and mixed spruce/ash) and at different stages of the forest cycle. Abundance cover of vegetation, dead wood and soil organic content were measured at each site. Ordinations of community structure revealed that spider assemblages were separated by both age and tree species across the forest cycle. The prethicket ash and spruce sites clustered together whereas in the older sites tree species had a greater effect. Mature ash sites formed a distinct group from the other sites. The presence of an adjacent ash stand did not affect the species richness of spruce plantations, although there were differences between the ash and spruce stands. Overall species richness was highest in the spruce and ash prethicket sites and the more open mature spruce sites. Mature ash sites had the lowest species richness. Lower field layer vegetation was positively correlated with species richness in the ash and spruce prethicket sites and the more open thicket and mature spruce sites. To enhance the biodiversity of open and forest species at a site level the growth of lower field layer vegetation should be encouraged at all stages of the forest cycle whilst retaining features typical of a mature forest. On a landscape scale, a mosaic of different aged plantations will provide adequate heterogeneity of habitat types in order to sustain both open and forest specialists and hence enhance biodiversity.