The dairy sector is the most important agricultural sub-sector in Northern Ireland both in terms of size and contribution to the economy. However, the abolition of the milk quota regime in 2015 and the government’s initiative for industry growth has given rise to concerns about environmental pollution, especially in terms of phosphorus (P) balance. In light of these concerns, this study analyses the level and determinants of P balance and use efficiency on Northern Ireland’s dairy farms. The study employs the OECD/ EUROSTAT nutrient balance methodology and the within-between farm random effects modelling technique on a balanced panel data set. The results show that P balance in dairy farms in the study area is relatively high. However, the more profitable dairy farms have relatively lower P balance. We also found that the amount of grass grazed, and fertilizer price have a negative and statistically significant relationship with P balance, while stocking density was found to have a positive relationship with P balance. On the other hand, the age of the farmer and the amount of grass grazed where found to have a positive and statistically significant relationship with P use efficiency. The study demonstrates that increasing the percentage of grass-based feeds reduces phosphorus surplus and also has a positive impact on the profitability of the dairy farms. The study recommends that farmers in the study area should increase the percentage of grass-based feeds alongside feeding concentrates with lower phosphorus contents. A moderate level of intensification should also be maintained.