Climate change scenarios can be used with crop simulation models to predict the impact of climate change on agricultural production. Focusing on two specific arable crops: (i) barley, a currently successful cereal crop, well adapted to the Irish climate and a staple of the tillage production sector; and (ii) potato, a traditional root crop in Ireland, sensitive to water stress and also a staple of the tillage production sector, the impact of climate change on yield was determined using simulation models (in the DSSAT package) and downscaled output from a general climate model. Daily weather data stochastically generated from mean monthly values for baseline (1961–1990), 2055 (2041–2060) and 2075 (2061–2090) climate periods were derived for over 500 locations in Ireland (derived from HADCM3Ga1). Important points with respect to the expected change in climate were that rainfall becomes more seasonally extreme, and a relatively uniform increase in temperature of about 1.6 ◦C over the country can be expected by the 2075 climate period. This change in climate is predicted to cause little change in the geographical distribution of barley yield, but grain yield in all areas is expected to increase with possibly a greater increase to the west. Potato yield in 2055 and 2075 is expected to fall for non-irrigated tubers. The impact is likely to be a severe loss of yield over most of the country by 2055. The implications of these findings are that barley will remain a viable cereal crop, and might find a greater role in livestock supplement feed supply (due to predicted drought losses effecting grass yield), but the irrigation demand for potato will be very significant, possibly making the crop non-viable for farmers, particularly in the east of Ireland where there will be competition for water in summer.