This paper describes the first cost-optimal assessment of national energy performance standards for buildings in Ireland undertaken in accordance with Article 5 of the Energy Performance of Buildings Directive (EPBD) Recast [Council Directive 2010/31/EU]. This paper focuses on new-build standards which are set out in Part L of the Building Regulations in Ireland. A set of representative residential and nonresidential building models were selected. The impact on primary energy demand of a wide range of energy efficiency measures and renewable technologies was evaluated for each building model and the corresponding lifecycle costs were calculated. The results show that the new-build residential standards in Ireland are in the cost-optimal range, while the new-build non-residential standards deliver a greater primary energy demand than the cost-optimal range.