A staff member in An Post is being investigated for the alleged theft of letters and small parcels.
Gardaí have confirmed they're looking into the incident in the Dublin 12 area.
A number of undelivered letters and small packages have since been found following an extensive investigation and are being sent out to customers.
An Post has apologised to those affected and described it as a "serious service failure".
The postal company has set up a a dedicated phone line for anyone affected, which can be contacted at 01 705 8600.
In a letter to affected customers, An Post apologised that standard mail "was not delivered in a timely fashion due to a serious service failure which is subject to an ongoing investigation by AnPost and An Garda Siochana".
It says the 'full resolution of this matter" and a return to "top-quality delivery service remain our highest priorities".
Newly-elected local TD Catherine Ardagh, who has a solicitors office in Crumlin, says there have been issues with the post since September.
"I noticed from a business point of view that our mail was coming sometimes really late, sometimes at four o'clock in the afternoon, or sometimes it wasn't coming for a few days.
"Then it would come and there would be old mail that should have maybe been delivered the previous week and stuff that we'd been waiting for".
