Cavan ("The Hollow") is the county town of County Cavan in Ireland. The town lies near the border with County Fermanagh in Northern Ireland. The town is on the main N3 road that links Dublin (to the south) with Enniskillen, Ballyshannon and Donegal Town (to the north).

Cavan was founded by the clan leader and Lord of East Breifne, Giolla Íosa Ruadh O'Reilly, between 1300 and his death in 1330. During his lordship, a friary run by the Dominican Order was established close to the O'Reilly stronghold at Tullymongan and was at the centre of the settlement close to a crossing over the river and to the town's marketplace. It is recorded that the (Cavan) Dominicans were expelled in 1393, replaced by an Order of Conventual Franciscan friars. The friary's location is marked by an eighteenth-century tower in the graveyard at Abbey Street which appears to incorporate remains of the original medieval friary tower. The imprint of the medieval town can be followed in the area of Abbey Street, Bridge Street and Main Street (townlands of Tullymongan Upper and Lower).

The O'Reillys later built a new castle in the late fourteenth century on Tullymongan Hill, overlooking the town centre. In the 15th century, the local ruler, Bearded Owen O'Reilly, expanded the town marketplace which attracted merchants from Dublin and Drogheda. The phrase "life of Reilly" is believed to derive from the great wealth and power of the O'Reilly clans, some of which came from their market. They also allowed counterfeit English and Scottish coins to be minted in their territory at this time.