Neither the ministry of the Irish Republic nor the Provisional Government had a deputy head of government , so the office of Vice-President of the Executive Council was a new idea of the Free State . Neither of the two administrations that preceded the Free State , the Ministry of the Irish Republic and the Provisional Government of Southern Ireland , included an office corresponding to deputy prime minister . In 1937 , when the new Constitution of Ireland came into force , the office of Vice-President of the Executive Council was replaced with that of TÃ naiste .