Three men and a woman have been arrested following the robbery of a cash-in-transit van.
Gardai also recovered a firearm and a large sum of cash after the incident outside Jobstown Post Office in Tallaght at around 12pm.
One man stole cash after threatening a male member of staff with what appeared to be a gun.
No shots were fired and the worker was not injured.
Later, gardai recovered a vehicle and searched a house at Cloonmore Green, also in Tallaght, where the weapon and cash were found.
The people arrested are aged in their mid to late 20s. They are being held at Tallaght and Rathfarnham garda stations.