The new Eircode system which gave every home and business in the country a postcode has moved airports, homes and towns to different counties.
The system that cost €27m was met with much head-scratching when it went live yesterday, and was also criticised for how it processed addresses in the Irish language.
But the Department of Communications has defended it, saying it will reduce confusion about the 35pc of addresses that are common or similar to neighbouring ones.
"It's not language-based like the car registration system, it's based on the existing system used by An Post which is why people might be less familiar with it," said a Department of Communications spokesperson.
While in Dublin there is some crossover in the initial three-digit Router Code, in that the old Dublin 1 becomes D01 and the old Dublin 24 becomes D24, outside the capital people are wondering why the letters and numbers bear no resemblance to their addresses.
Areas of Sligo have been designated the letter F, and in other places the letters in the codes do not conform with the names of their counties.
Patrick Edmond, the managing director for the inter- national aviation services centre at Shannon Airport, was one of the first to notice something odd.
"Amazing what new precise Eircode system can do. Shannon Airport has moved from Clare to Limerick," he tweeted.
Someone else took to Twitter to say his address was listed as Ennis under the Eircode system even though the town is around 30km away from where he lives.
Another person they now appeared to be living in Waterford even though their address is in Kilkenny.
Enniscrone in Co Sligo appears to have moved to neighbouring Co Mayo, and some people in Celbridge, Co Kildare, said they now apparently live in Naas.
On top of this, in many rural areas, cowsheds and barns have been given their own postcodes despite the fact that some buildings have been derelict for years.
Users also said the system spells names incorrectly.
Examples of Lahinch and Ennistymon were given as being spelled as Lehinch and Ennistimon.
But the Department of Communications said people's addresses are not changing.
"The Eircode system is based on the An Post system which is governed by sorting office locations and that is why a very small number of addresses appear different," said a spokesperson.
"If you look up a building in Shannon Airport on the An Post website it gives you Limerick as well - the addresses are the same on both the An Post and Eircode systems."