Northern Ireland continues to use the national system initiated for the whole of the United Kingdom of Great Britain and Ireland in 1903, with two-letter county and city codes featuring the letters I or Z representing Ireland. The full list of codes appears below.his is despite the fact that Northern Ireland, while part of the United Kingdom, is not part of Great Britain. From 21 July 2014, vehicle registration in Northern Ireland became the responsibility of the DVLA in Swansea.