Between 200 and 300 post offices could be forced to close over the next 18 months unless they are granted access to government services.
The Irish Postmasters Union says a deal agreed with An Post last year, provided for the introduction of services, such as car tax, drivers' licenses and Leap cards.
However, it says in the time since, no progress has been made.
Speaking ahead of their annual conference in Portlaoise later, IPU General Secretary Ned O'Hara says the issue could lead to hundreds of post offices having to close their doors.
"An Post have introduced a range of new services which we have adapted to and financial services like credit cards and loans etc," said Mr O'Hara.
"But we haven't had any sign of any government services yet and they are critical to the post office service going forward.
"So, unless the network gets those services going forward, within a year or 18 months 200-300 post offices will be at risk."