The union says a report by Grant Thornton has found that 27 million euro could be saved over five years by the move, because of transaction costs.
Currently motor tax renewals are offered by county offices.
The Irish Postmasters Union says relieving those offices of the service would save taxpayers' money and help local post offices.
Ned O'Hara is the IPU Acting General Secretary.
“Well there's a current whole of government approach in relation to this sustainability of the post office network and it was initiated in March 2013 – and it is to ensure the survival of the post office network” he aid.
“There is a whole of government approach in relation to security extra business for the post office to survival of the post office network throughout the country” he added.
