The young woman had a handgun pointed at her when two men raided a shop in Porthall, Lifford.
The woman was in the shop at around 7.15pm when the two men arrived and threatened her.
The demanded cash from the cash register and also demanded the keys of her car – a black BMW 316.
The two men made their getaway at high-speed towards Strabane and eye-witnesses said they later saw the car cross the Lifford Bridge at speed.
The woman was left shocked but was not hurt following her ordeal.
No shots were discharged during the raid in which a small amount of cash was stolen.
A Garda spokesman has appealed for anyone who was in the vicinity of Porthall in Lifford between 6pm and 7.30pm on Wednesday evening to contact them.
“This was a very serious incident and the young woman was left very shaken after her ordeal.
“We are liaising with the PSNI as we believe the stolen car may have been driven across the border.
“Anybody who may have noticed anybody acting suspiciously around Porthall in Lifford on Wednesday evening is asked to contact Gardai,” he said.