This paper outlines how an existing energy system can be transformed into a 100% renewable energy system. The transition is divided into a number of key stages which reflect key radical technological changes on the supply side of the energy system. Ireland is used as a case study,but in reality this reflects many typical energy systems today which use power plants for electricity, individual boilers for heat, and oil for transport. The seven stages analysed are 1) reference, 2) introduction of

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... istrict heating, 3) installation of small and large-scale heat pumps,4) reducing grid regulation requirements, 5) adding flexible electricity demands and electric vehicles, 6) producing synthetic methanol/DME for transport, and finally 7) using synthetic gas to replace the remaining fossil fuels. For each stage, the technical and economic performance of the energy system is calculated. The results indicate that a 100% renewable energy system can provide the same end-user energy demands as today's energy system and at the same price. Electricity will be the backbone of the energy system, but the flexibility in today's electricity sector will be transferred from the supply side of the demand side in the future. Similarly, due to changes in the type of spending required in a 100% renewable energy system, this scenario will result in the creation of 100,000 additional jobs in Ireland compared to an energy system like today's. These results are significant since they indicate that the transition to a 100% renewable energy system can begin today, without increasing the cost of energy in the short- or long-term, if the costs currently forecasted for 2050 become a reality.