a barony in Donegal was in 1851 when the barony of Inishowen was divided into Inishowen East and Inishowen West. The barony of Kilmacrennan covers a large portion of northwest Donegal and is the largest in the county. With an area of , Kilmacrennan is also the largest barony in Ireland, being roughly equal in size to County Monaghan. The smallest barony is Inishowen West, at . Townlands are the smallest officially defined geographical divisions in Ireland. There are 2,787 townlands in Donegal and an additional 47 historic town boundaries. These town boundaries are registered as their own townlands and are much larger than rural townlands. The smallest rural townlands in Donegal are just 1 acre in size, most of which are either lough islands or offshore islets (Corragh Island, Bishop's Island, Juniper Island, O'Donnell's Island, etc.). The largest rural townland in Donegal is 6,053 acres (Tawnawully Mountains). The average size of a townland in the county (excluding towns) is 438 acres. Informal districts The county may be informally divided into a number of traditional districts. There are two Gaeltacht districts in the west: The Rosses (), centred on the town of Dungloe (), and Gweedore (). Another Gaeltacht district is located in the north-west: Cloughaneely (), centred on the town of Falcarragh (). The most northerly part of the island of Ireland is the location for three peninsulas: Inishowen, Fanad and Rosguill. The main population centre of Inishowen, Ireland's largest peninsula, is Buncrana. In the east of the county lies the Finn Valley (centred on Ballybofey) and The Laggan district (not to be confused with the Lagan Valley in the south of County Antrim), which is centred on the town of Raphoe. Physical geography The county is the most mountainous in Ulster consisting chiefly of two ranges of low mountains; the Derryveagh Mountains in the north and the Blue Stack Mountains in the south, with Errigal at the highest peak, making it the 11th-highest county top in Ireland. It has a deeply indented coastline forming natural sea loughs, of which Lough Swilly and Lough Foyle are the most notable. Donegal boasts the longest mainland coastline of any county in Ireland, and has either the longest or third longest total coastline (including islands), depending on measurement. Length estimates of the Donegal coastline range from less than to . The official figure used by Donegal County Council is . The Slieve