Letterkenny, nicknamed the Cathedral Town, is a large town in County Donegal, Ireland, on the River Swilly in the north-west of Ulster. Along with the nearby city of Derry, Letterkenny is a regional economic gateway for the north-west of Ireland. Letterkenny began as a market town at the start of the 17th century, during the Plantation of Ulster. A castle once stood near where the Cathedral of St Eunan and St Columba, County Donegal's only Catholic cathedral, stands today.