Castlewellan (from Irish Caisleán Uidhilín 'Hugelin's Castle') is a small town in County Down, in the south-east of Northern Ireland close to the Irish Sea. It is beside Castlewellan Lake and Slievenaslat mountain, 11 miles (18 km) southwest of Downpatrick. It lies between the Mourne Mountains and Slieve Croob. It had a population of 2,782 people in the 2011 census.