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This study compared the profitabilities of systems of dairy production based on N-fertilized grass (FN) and grass-white clover (WC) grassland and assessed sensitivity to changing fertilizer N and milk prices. Data were sourced from three system-scale studies conducted in Ireland between 2001 and 2009. Ten FN stocked between 2·0 and 2·5 livestock units (LU) ha?1 with fertilizer N input between 173 and 353 kg ha?1 were compared with eight WC stocked between 1·75 and 2·2 LU ha?1 with fertilizer N input between 79 and 105 kg ha?1. Sensitivity was confined to nine combinations of high, intermediate and low fertilizer N and milk prices. Stocking density, milk and total sales from WC were approximately 0·90 of FN. In scenarios with high fertilizer N price combined with intermediate or low milk prices, WC was more (P < 0·05) profitable than FN. Based on milk and fertilizer N prices at the time, FN was clearly more profitable than WC between 1990 and 2005. However, with the steady increase in fertilizer N prices relative to milk price, the difference between FN and WC was less clear cut between 2006 and 2010. Projecting into the future and assuming similar trends in fertilizer N and milk prices to the last decade, this analysis indicates that WC will become an increasingly more profitable alternative to FN for pasture-based dairy production.