A modelling exercise was conducted using the Pasture Base Herd Dynamic Milk model on 12 years weather data at three locations in Ireland: Johnstown Castle (south east), Ballyhaise (north) and Oak Park (centre). Simulations were run on a 40 ha farm. There were two soil types: free draining soil (FDS) or heavy soil (HS). Chemical nitrogen (N) fertiliser application was 225 kg ha–1 annually. Concentrate feeding was fixed at 480 kg DM cow–1 year–1 and only grazed grass and grass silage were fed to the animals. Simulations were optimised in terms of stocking rate (SR) to ensure forage self-sufficiency. Grass growth, start and end of the grazing seasons, grass and silage intake and total grazing days were outputs from the model. Results show that the ideal SR was dependent on the interaction between soil type and weather. The worst combination was Ballyhaise weather combined with the HS, which could only sustain a SR of 2.4 cow ha–1 at 225 kg N ha–1. This was not due to a lower grass growth but to a lower number of grazing days per cow as soils were too saturated to facilitate grazing.