▁which ▁house . ▁ ▁Ireland ▁was ▁the ▁last ▁country ▁in ▁the ▁O E CD ▁to ▁create ▁a ▁post code ▁system , ▁which ▁is ▁called ▁E ir code . ▁ ▁In ▁July ▁ 2 0 1 5 ▁all ▁ 2 . 2 ▁million ▁resident ial ▁and ▁business ▁addresses ▁in ▁Ireland ▁received ▁a ▁letter ▁not ifying ▁them ▁of ▁the ▁new ▁E ir code ▁for ▁their ▁address . ▁Un like ▁other ▁countries , ▁where ▁post codes ▁define ▁clusters ▁or ▁groups ▁of ▁addresses , ▁an ▁E ir code ▁ident ifies ▁an ▁individual ▁address ▁and ▁shows ▁exactly ▁where ▁it ▁is ▁located . ▁The ▁system ▁was ▁critic ised ▁at ▁its ▁launch . ▁ ▁Res p ons ibility ▁for ▁the ▁current ▁postal ▁delivery ▁system ▁rest s ▁with ▁An ▁Post , ▁a ▁semi - state ▁body ; ▁however , ▁the ▁Department ▁of ▁Communic ations , ▁Energy ▁and ▁Natural ▁Resources ▁( DC EN R ) ▁retain s ▁the ▁right ▁to ▁reg ulate ▁addresses ▁if ▁they ▁wish ▁so . ▁ ▁Pre - E ir code ▁postal ▁districts ▁ ▁Dublin ▁ ▁In ▁Dublin ▁city ▁and ▁its ▁sub ur bs , ▁a ▁system ▁of ▁postal ▁districts ▁was ▁introduced ▁in ▁ 1 9 1 7 ▁by ▁the ▁Royal ▁Mail ▁with ▁the ▁prefix ▁" D ", ▁and ▁retained ▁after ▁Ireland ▁became ▁an ▁independent ▁country , ▁without ▁the ▁prefix . ▁However ▁the ▁use ▁of ▁district ▁numbers ▁by ▁the ▁public ▁did ▁not ▁begin ▁until ▁ 1 9 6 1 , ▁when ▁street ▁signs ▁displayed ▁postal ▁district ▁numbers . ▁Prior ▁to ▁that ▁time , ▁street ▁signs ▁only ▁displayed ▁the ▁street