Letterkenny , known as the Cathedral Town, is the largest and most populous urban settlement in County Donegal. Its English name is derived from the Irish name Leitir Ceanainn, meaning "Hillside of the O'Cannons" – the O'Cannons being the last of the ancient chieftains of Tír Conaill. With a population of 19,588, Letterkenny is located on the River Swilly in east Donegal. Along with the nearby city of Derry, Letterkenny forms the major economic core of the north-west of the island of Ireland.