Captain Crawford received a grant of 1,000 acres during the Plantation of Donegal, and near his house, Sir George Merbury established what would become Letterkenny; a collection of 50 thatched houses, with only 13 boasting the relative luxury of clay and stone walls, plus a single watermill. By 1654, the town had grown considerably more sophisticated, featuring a large stone dwelling house protected by a bawn (a defensive wall) with four flankers, a “fair church”, and a bridge spanning the River Swilly at the town’s edge. Every Friday, locals and visitors alike would gather for the weekly market, whilst twice yearly fairs brought traders from further afield.

The town’s importance was officially recognised in the Down Survey of 1655-6, which listed Letterkenny as one of three “Considerable Townes” in the barony of Kilmacrenan. With its own constable and stocks for punishing wrongdoers, the settlement likely housed a gaol somewhere within its bounds. The heart of this historic town can still be traced through modern Letterkenny’s street layout and telling placenames; Castle Street once led directly from the Church of Ireland (built on the site of the original 17th century parish church) down to the plantation castle near Mount Southwell’s southern corner.

Today, echoes of the plantation era persist in names like Fort Well and Castle Gardens, which mark the probable location of the original fortified house and bawn. A large two-storey house now stands beside Castle Gardens, potentially built atop the castle’s foundations, whilst substantial stone walls surrounding the property may incorporate remnants of the 17th century defences. The nearby Conwal church and graveyard, though rebuilt in the 19th century, still shelter graveslabs dating back to the 1600s, silent witnesses to Letterkenny’s transformation from a collection of thatched cottages into the thriving town it would become.