Recent ecological research provides evidence that an increased number of plant species in natural grasslands is associated with increased biomass productivity, and provides a wide range of other ecosystem benefits. This suggests that increases in species diversity in agricultural ecosystems may similarly lead to increased benefits. The work reported below was part of the COST 852 Agrodiversity experiment, carried out at 34 sites across Europe. In Ireland, the effects of four-species grass-clover mixtures on herbage production, species persistence and unsown species suppression at three sites over multiple years, were investigated under growing conditions that were intensive relative to unfertilised natural grassland systems. The design included a range of four-species mixtures and monocultures of perennial ryegrass, timothy, cocksfoot, white clover, red clover and Caucasian clover. Several harvests were taken at each site for two or three years. Species diversity had a strong, persistent and positive effect on overall yield and the yield of sown species, and enhanced resistance to the growth of unsown species. Mixtures generally yielded well when compared with the best monoculture, and sometimes out yielded it. These effects on total yield declined over time but were still important at the end of the experiments. The diversity effects on sown species yield and on resistance to unsown species increased with time. Diversity effects were robust to changes in species composition, and persisted for the duration of the experiments across mixtures and over time. Virtually every mixture had a higher yield, and suppressed unsown species better, than monocultures of perennial ryegrass. These patterns were broadly consistent across sites. The persistence of species varied widely and was not consistent across sites.