County Monaghan is a county in Ireland. It is part of the Border Region and is in the province of Ulster. It is named after the town of Monaghan. Monaghan County Council is the local authority for the county. The population of the county is 60,483 according to the 2011 census.
In 1585, the English lord deputy of Ireland, Sir John Perrot, visited the area and met the Irish chieftains. They requested that Ulster be divided into counties and land in the kingdom of Airgíalla be apportioned to each of the McMahon chiefs. A commission was established to accomplish this and County Monaghan came into being. The county was subdivided into five baronies: Farney, Cremorne, Dartrey, Monaghan and Truagh, which was left under the control of the McKenna chieftains.
After the defeat of the rebellion of Hugh O'Neill, The O'Neill and the Ulster chieftains in 1603, the county was not planted like the other counties of Ulster. The lands were instead left in the hands of the native chieftains. In the Irish Rebellion of 1641 the McMahons and their allies joined the general rebellion of Irish Catholics. Following their defeat, some plantation of the county took place with Scottish and English families.
For more information, see the EN Wikipedia article County Monaghan. especially the section "Geography and political subdivisions" and its subsections "Baronies", "Civil parishes and townlands" and "Towns and villages"