A masked man, described as 5 foot 10 inches, entered the premises at around 7.45pm threatening staff.
He escaped in a green coloured UK registered vehicle driven by a second man.
No one was physically injured in the robbery but a undisclosed sum of money was taken.
Gardai are viewing CCTV footage as part of the investigation - they are also asking anyone who may have seen anything suspicious in the Ballyduff Road area of Lifford between 7pm and 8pm yesterday evening to contact Gardai.
The Lifford incident comes a day after the home of two elderly sisters was targeted nearby - Local Councillor Gerry Crawford says locals thought these type of incidents had died down: