Letterkenny (Irish: Leitir Ceanainn Irish pronunciation: [ˈl̠ʲɛtʲəɾʲ ˈcanˠən̠ʲ], meaning "hillside of the O'Cannons"), 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. Letterkenny Castle, built in 1625, was located south of Mt Southwell on Castle Street. County Donegal's largest third-level institution, Atlantic Technological University (ATU) Letterkenny, is located in the town, as are St Eunan's College, Highland Radio, and a Hindu temple. Letterkenny was also the original home of Oatfield Sweet Factory, a confectionery manufacturer; the factory closed and was demolished in 2014. In 1798, Theobald Wolfe Tone was arrested at Laird's Hotel in the town. In 2015, Letterkenny was judged as the tidiest town in Ireland.

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