The price of a standard domestic stamp is set to increase from €1 to €1.10 from next month.
A standard international stamp will also increase by 10c to €1.80.
The last price increase was in April 2017 and An Post said the new increase reflects other European postal services' price increases.
The company said that increases in letter prices across Europe reflect the steady decline in traditional letter volumes globally "due to e-substitution".
Existing stamps will remain fully valid after 19 March and any additional postage may be purchased at any post office (e.g. a 10c stamp to make up the difference).
Furthermore, all stamps bearing the 'N' denomination will automatically represent €1.10 postage from 19 March.
Similarly, all 'W' stamps will cover the new €1.80 international letter rate.