Nitrous oxide (N2O) emissions associated with urine nitrogen (N) deposition during grazing are a major component of greenhouse gas emissions from domestic livestock. The present study investigated the relationship between urine N loading rate and the efficacy of a nitrification inhibitor, dicyandiamide (DCD), on cumulative N2O emissions from a grassland soil in Ireland over 80 and 360-day periods in 2009/10 and 2010/11. A diminishing curvilinear relationship between urine N rate and cumulative N2O emissions was observed in both years. Despite this increase in cumulative N2O emissions, the emission factor (EF3) for N2O decreased with increasing urine N rate from, on average, 0·24 to 0·10% (urine applied at 300 and 1000 kg N/ha, respectively), during an 80-day measurement period. This was probably the result of a factor other than N, such as carbon (C), limiting the production of N2O. The efficacy of DCD varied with urine N loading rate, and inter-annual variability in efficacy was also observed. Dicyandiamide was effective at reducing N2O production for 50–80 days after urine application, which accounted for the major period of elevated daily flux. However, DCD was ineffective at reducing N2O production after this period, which was likely a result of its removal from the soil via degradation and leaching.