 Complete the following phrase:  Customs controls were introduced on the frontier on 1 April 1923, shortly after the establishment of the Irish Free State. These controls were maintained, with varying degrees of severity, until 1 January 1993, when systematic customs checks were abolished between European Community member states as part of the single market. There are no longer any operational customs posts along either side of the border. Except during a brief period during World War II, it has never been necessary for Irish or British citizens to produce a passport to cross the border. However during the 1970s 