The workload in city post offices has been steadily increasing over the last few years, but income has been "dropping to hell", said one postmaster in a city-centre sub-post office.
He said an increase in the population meant more people were using the post office to pay bills, collect social welfare and pay corporation charges, among other things.
At the same time, income was falling in sub-post offices. The main reasons, he said, were the payments system and post offices no longer setting franking machines.
Previously, post offices would set machines manually and would be paid per amount of stamps they set. "The income from these machines is gone," he said.
An Post says the number of franking machines being set is declining all the time, as manufacturers have developed faster means of doing the work.
The scale of payments system also meant the more work done the less payment per unit received.
An Post said the system meant sub-post offices, which were An Post agents, were paid a salary based on transactions involved. This meant a postmaster would not get as much for the 5,000th transaction as for the fifth.
The postmaster said the fall in income was compounded by rising rents, especially in Dublin, Limerick, Galway, Cork and Waterford.
His sub-post office in the city, one of the top 400 in terms of turnover, was run alongside another business, which slightly subsidised the post office.
Like most other sub-post offices in towns and cities, dual staffing was in operation. Staffing shortages were also a problem, he said, but it was often not financially viable to hire more staff.
Other postmasters have shut down the business altogether, he said. And people who would like to get into the business were put off by the low financial return.
Government proposals on post offices were "too late for a lot of us. They should have happened 20 years' ago," he said. It was a pity, he said, because "the post office was the first franchise, not McDonalds".