An Post is warning deliveries in some areas are being impacted by staffing shortages.
The company says it is "currently experiencing a significant increase in COVID-19 related absence".
In a statement, it says this is "placing a temporary strain on our resources", which may impact on regular delivery levels across its network.
It says some post offices may also be affected.
"We have a range of contingency plans in place, and our staff and postmasters are working hard to minimise the impact on customers", it adds.
"We apologise for any inconvenience this situation may cause. The health and safety of our staff and customers is our priority."
But it says where any post office has to close temporarily, social welfare payments will be transferred to a neighbouring office.
An Post saw revenues grow by 2.6%, according to its 2020 annual report.
But it said strong parcel growth of 100% was "partially offset by a decline in letter volumes" of 7% - as well as trading declines in post offices due to pandemic restrictions.
While the company says it saw "a very strong start" to 2021, reflecting continued high parcel volumes and a recovery in letter volumes.
And in November 2020, the postal service saw parcel volumes exceed three million per week due to a surge in online shopping.
It said the mix of store closures due to COVID restrictions, and online retail offers, saw a 230% increase in parcels when compared to the same period in 2019.