The van did not contain any money but another postal van carrying social welfare cheques had passed through the area a short while before the raid.
The gang had blocked the main Dublin to Derry road between Belturbet and Cavan with a lorry.
At 7.25am an An Post van was stopped and the gang forced the driver to drive up a nearby laneway and then blocked that road with a lorry.
The gang demanded the driver hand over money that they believed was in the van.
After discovering that there was no money, the gang smashed up the van using hammers and removed some post that had been destined for the Ballyhaise area.
As they fled they set fire to the van they had used. They poured petrol over the vehicle but did not set it alight.
Registered and ordinary post destined for the Ballyhaise area is missing and may have been destroyed in the fire.
It is believed some post for Ballyconnell, Swanlinbar and Belturbet may also have been destroyed.
A spokesperson for An Post said the driver was physically uninjured but shaken by the incident.
Gardaí are investigating the incident.
