The team of tutors and students from the University of Sunderland made their discovery last week during a 10-day field trip to Culdaff on the Inishowen peninsula.
Using the latest in mapping equipment, they discovered a circular boundary wall, some 100 metres in diameter, buried underground in fields at Carrowmore.
The location of the find has two high crosses and is already known as an early Christian site
dating back to the sixth century.
However, the latest discovery provides the first physical proof that an early medieval
monastery existed at the spot.
Archaeologist and group co-leader, Colm O'Brien, said the discovery was "beyond (their) wildest dreams".
"For the first time, we can see the precinct boundaries of a monastery at Carrowmore, and we therefore have a context within which to understand those high crosses," he said.