The price of a stamp is set to jump to €1, and a "significant number" of post offices will close.
Head of An Post David McRedmond also warned of a large number of job losses in the semi-State company.
There are now fears of large-scale closures from the 1,300 post offices in the company's network.
Mr McRedmond said the price of stamps will go from 72c to €1.
This is a rise of almost 40pc, one of the sharpest increases in the price of a stamp in years.
It is the third rise in the price of postage stamps in the past two years. The last increase took effect in July.
Experts said the rise of 28c in the cost of sending a letter will further discourage people from using the postal system, pushing more people to use email.
An international stamp will rise from €1.10 to around €1.35, a rise of 23pc. Business users on a typical bulk rate will see prices of stamps go up from 50c to 57c.
Higher prices for postage are expected to be implemented from mid-April.
Communications Minister Denis Naughten defended the huge rise in the cost of sending a letter.
He said TDs had voted to lift the price cap on the price of stamps to help An Post become viable.
"The purpose of the legislation is to protect the five-day delivery service," he said.
The move comes as mail volumes have fallen heavily. The latest annual report for An Post shows mail volumes fell by 35pc in 2015 compared with a peak in 2007.
Mr McRedmond warned that a large numbers of postal workers are set to lose their jobs.
His comments come after rural groups reacted with fury to a recommendation in a report for the Government that large numbers of post offices may have to close.
A group devising a future strategy for An Post, chaired by businessman Bobby Kerr, has said 80 offices may have to close.
Asked if 80 post offices, mainly in rural areas, would be shut down, Mr McRedmond would not put a number on it.
"There will certainly be a significant number of closures," he said on the 'Today With Sean O'Rourke' programme.
"There are far too many of them," he added.
Mr McRedmond said that after the post office closures there will still be a large number of branches per head of the population.
An Post is currently considering the Kerr Report and the number of closures in conjunction with consultants McKinsey.
An Post's board will decide on this in the next few weeks, he said.
The emphasis will be on co-locating post offices by providing post offices in supermarkets, he said.
He revealed that An Post is losing €10m a year, but denied reports that the semi-state company would not be able to pay its 9,000 staff next month.
He said the overall staff numbers in An Post have fallen by 1,700 in the last decade, and said a similar reduction was likely in the next few years.
Mr McRedmond revealed that the Kerr Report recommends a State bailout of €58m for An Post, but he said the postal company is not seeking a bailout.