At around 10pm on Sunday, a man armed with a knife, entered the shop in the Donaghmore Road area and demanded cash from a worker.
A sum of money was handed over and the robber fled on foot towards Quarry Lane.
The staff member was left badly shaken by the incident.
The man is described as being around 20 years old, approx 6" tall and was wearing a dark blue jacket, a light blue hooded top, blue jeans and a pink balaclava.
Two men are in custody helping police with their inquiries.
Police have asked anyone who witnessed this incident or noticed any suspicious activity in the area to contact them.