Delays of up to a week have forced An Post to bring forward latest dates for posting in advance of Christmas for e-commerce retailers.
The move comes as the company's capacity to process parcels comes under severe pressure due a large increase in demand.
In an email to commercial customers and retailers this week, the company said that the action is necessary "in the interest of fulfilling customers' expectations".
An Post says that delivery by Christmas Day cannot be guaranteed for parcels posted by retailers after Monday 14 December.
It means that last dates for posting for those fulfilling online orders have been brought forward by nearly a week.
An Post said that it was "an extremely difficult" decision to make, adding that it knows retailers and online sellers will be dissatisfied by it.
However, it explained to customers that it had no other option and it was making the change in order to avoid disappointed to consumers.
An Post has seen a 230% increase in parcel deliveries.
During November, the company delivered 2.5 million parcels every week, it expected that to increase to 3.5m in the coming weeks.
The move will have an impact on online sales as people weigh up expected delivery times.
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In a statement issued to RTÉ News, An Post said the revised deadline applies only to its "bigger e-commerce retailers and shippers and not to ordinary customers posting parcels and packages for whom the advised latest dates still apply".
Consumers are advised to post Christmas cards to the US by Friday 11 December. Cards to Europe should be posted by 17 December and EU parcels by 17 December.
For cards posted to the UK the posting date to be sure of delivery of cards and parcels in time for Christmas is 18 December.
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Commenting on the change to post by dates for online retailers and commercial customers the company said: "We prefer to be upfront so that these major online sellers can plan their operations and advise their customers accordingly."
Garrett Bridgeman, the Managing Director of Mail and Parcels with An Post, said there are no changes to dates for ordinary customers posting cards and parcels, or for those who have ordered items from small to medium sized businesses.
Speaking on RTÉ's News at One, Mr Bridgeman said the company's capacity to process parcels is coming under pressure due to a large increase in demand.
He said An Post is currently processing three million parcels a week, compared to one million parcels a week last year.
For customers ordering online from a large retailer, Mr Bridgeman said: "Our advice is to make the purchase by the 14th to ensure you have it by Christmas Day.
"Chances are that items bought online after the 14th will probably be delivered before Christmas Day, but we all know at Christmas time that 'probably' isn't good enough, especially for purchases you need by Christmas morning."
