An Post says some services are "under quite a bit of pressure" currently, with over 1,000 staff across the company absent due to COVID-19.
The company says the current situation may impact on their usual mail and parcel delivery levels.
Four post offices - mostly smaller sites in Cork, Waterford, Leitrim and Clare - have also had to temporarily close due to staffing issues, with social welfare payments transferred to nearby post offices instead.
It comes as record numbers of COVID-19 cases have left tens of thousands of people nationally isolating as either confirmed cases or as close contacts.
Anna McHugh, head of corporate communications at An Post, told The Pat Kenny Show they're facing the same challenges many other businesses and service providers are dealing with during the Omicron wave.
She said: “We’ve certainly had a very challenging time since Christmas, and that’s continuing this week.
“That said, we’re really pulling out all the stops across the country - postmasters and staff [alike]. We have just four post offices closed today - we’d hope two of those would be open by mid-week.”
Other post offices are closing a little earlier in the day to allow staff to safely cover shifts as needed.
Ms McHugh said the situation is changing by the hour, but for now many services are feeling the strain.
She explained: “We’re looking at almost 1,100 staff out nationally - that’s between headquarters, mail centres, post offices and of course the delivery staff.
“There are some areas that are under quite a bit of pressure."
Parcel services are also busy with increased deliveries from the January sales, while An Post is also dealing with the extra customs requirements and complications post-Brexit.
Despite that, Ms McHugh said staff are used to working in a "COVID environment" - having done so for nearly two years now.
She noted: “Some of our staff haven’t seen their colleagues for almost two years, as they might be in at 6:30am and gone before the next pod comes in at 7:30am.”
Staff who are moving around - such as the tech experts keeping An Post's various systems up and running - do need to use antigen testing “all the time”.
Meanwhile, staff have been continuing to deliver HSE antigen tests to close contacts and suspected cases right through the weekend - with over 1,000 staff on duty on both Saturday and Sunday.