I think that their service is massively overwhelmed and either understaffed or poorly staffed.
An Post took on the delivery fulfillment contracts for Vinted, Shein and, I think, Temu/Ali Express a while back. These retailers, perhaps not so much Vinted, but the Chinese ones are absolutely pumping shipments (most of which are low value and low quality products) into Ireland.
Add to that the fact that An Post is also a delivery partner of tertiary sellers on Amazon (inc. World Of Books, Music Magpie, Awesome Books etc.).
The service is flooded with packages and it's jamming up deliveries.
I recently ordered a new credit card, the bank said it would take 1 - 3 working days. 8 days later the bank rang me asking why I hadn't activated my card - still hadn't received it.
EDIT: The other thing is - Fastway went to the wall a while back so now, as a Joe/Jane Soap sending a package, you only have the likes of An Post/GLS as an option for low-cost couriers in Ireland thus adding to the bottleneck.