Greencastle is a borough in Franklin County in south-central Pennsylvania, United States. The population was 3,722 at the 2000 census. According to the United States Census Bureau, the borough has a total area of 1.6 square miles (4.1 km2), all land.Greencastle was founded in 1782 by John Allison. The town was named either after a small town in County Donegal, Ireland, a suburb of Belfast of the same name in County Antrim, or Greencastle, County Down or possible in consideration of all as the local area has place names of both County Antrim and County Donegal and was settled by Scotch-Irish settlers. It was originally composed of 246 lots. By 1790 there were about sixty houses in Greencastle, homes to approximately 400 people. The town of Greencastle had grown by the mid-nineteenth century to 1,125 residents.