The Association of Garda Sergeants and Inspectors (AGSI) said it is concerned at the idea of greater devolution to regional Garda forces.
In a statement released today AGSI said the creation of a more autonomous regional structure for the gardaí could create additional bureaucracies and different styles of policing in different areas.
"The national model has worked well, by and large, and we should be mindful of the experiment with regional health boards which was not a happy one," Joe Dirwan, General Secretary AGSI, said.
AGSI was responding to recommendations in the latest report from the Garda Inspectorate published yesterday.
In the report the Garda Inspectorate recommended that accountability should be devolved to the six Garda regions, each headed by assistant commissioners, who would be made fully responsible for all aspects of policing in their areas.
AGSI also said it will oppose any attempt to introduce "accelerated" promotions.
"Any such cherry-picking of persons for fast-track promotions could put us right back to the bad old days when nepotism was rife and there was a huge perception that promotions and appointments were made on a favouritism basis and not on merit," Mr Dirwan said.
However, the union said it is supportive of other recommendations mentioned in the report including; more civilian support for frontline policing, the increase in technological aids, and the need for better police vehicles.
Mr Dirwan said he will meet the Chief Inspector in the near future to discuss the issues raised in the report.
Meanwhile, this evening the Inspectorate responded to the AGSI by saying it is pleased to note the union is generally supportive of the recommendations made in the report and that they only highlighted two concerns.
The Inspectorate denied devolving decision-making within the force would increase bureaucracies and different styles of policing in different areas. It also said it is against fast-tracking promotions.
In yesterday's report the Inspectorate said: "It is crucial to get the most qualified Gardaí to sergeant and subsequent ranks as soon as they are appropriately prepared to assume the requisite responsibilities."
However, the Inspectorate say they made this recommendation because it believes it is "important to avoid circumstances in the future in which significant experience, expertise and corporate knowledge are lost from the Garda Síochána through a series of retirements of senior officers in and around the same time".