The GPO in Dublin was closed on Thursday afternoon following a public order incident.
A man, who is in his 50s, was detained after he was armed with a knife and he is currently being held at a garda station in the city centre.
No members of staff or customers were injured during the incident.
Gardaí carried out forensic examinations at the scene and say their investigation is ongoing.
The GPO will reopen as normal on Friday.
