The Paris Agreement is the last hope to keep global temperature rise below 2°C. The consensus agrees to holding the increase in global average temperature to well below 2°C above pre-industrial levels, and to aim for 1.5°C. Each Party’s successive nationally determined contribution (NDC) will represent a progression beyond the party’s then current NDC, and reflect its highest possible ambition. Using Ireland as a test case, we show that increased mitigation ambition is required to meet the Paris Agreement goals in contrast to current EU policy goals of an 80–95% reduction by 2050. For the 1.5°C consistent carbon budgets, the technically feasible scenarios' abatement costs rise to greater than €8,100/tCO2 by 2050. The greatest economic impact is in the short term. Annual GDP growth rates in the period to 2020 reduce from 4% to 2.2% in the 1.5°C scenario. While aiming for net zero emissions beyond 2050, investment decisions in the next 5–10 years are critical to prevent carbon lock-in. Key policy insightsEconomic growth can be maintained in Ireland while rapidly decarbonizing the energy system.The social cost of carbon needs to be included as standard in valuation of infrastructure investment planning, both by government finance departments and private investors.Technological feasibility is not the limiting factor in achieving rapid deep decarbonization.Immediate increased decarbonization ambition over the next 3–5 years is critical to achieve the Paris Agreement goals, acknowledging the current 80–95% reduction target is not consistent with temperature goals of ‘well below’ 2°C and pursuing 1.5°C.Applying carbon budgets to the energy system results in non-linear CO2 emissions reductions over time, which contrast with current EU policy targets, and the implied optimal climate policy and mitigation investment strategy. Economic growth can be maintained in Ireland while rapidly decarbonizing the energy system. The social cost of carbon needs to be included as standard in valuation of infrastructure investment planning, both by government finance departments and private investors. Technological feasibility is not the limiting factor in achieving rapid deep decarbonization. Immediate increased decarbonization ambition over the next 3–5 years is critical to achieve the Paris Agreement goals, acknowledging the current 80–95% reduction target is not consistent with temperature goals of ‘well below’ 2°C and pursuing 1.5°C. Applying carbon budgets to the energy system results in non-linear CO2 emissions reductions over time, which contrast with current EU policy targets, and the implied optimal climate policy and mitigation investment strategy.