There were no customers in the premises at the time and no one was injured during the raid.
A third robber sat outside in what is believed to be a large silver Volvo car in which all three made their getaway across the border.
Buncrana based Inspector David Murphy said the car pulled up outside the Post Office around 12.20 carrying three masked men. Two of them entered the premises, one carrying a fire arm.
Staff were ordered at gunpoint to hand over money and made off in the Volvo.
“We are interested to hear from any witnesses about the movement of the silver Volvo before and after the robbery. We are working closely with the PSNI and talking to witnesses who were outside the Post Office at the time of the robbery,” Inspector Murphy said.
Anyone with information can contact gardai at Buncrana on 174 9320540 or Burnfoot Garda Station on 074 93 68194 or use the confidential line on 1800 666111.