County Down is one of the six counties of Northern Ireland and is bounded by County Antrim to the north; Strangford Lough, the Ards Peninsula and the Irish Sea to the east; County Armagh to the west; and Carlingford Lough to the southwest. The River Lagan forms most of the border with County Antrim and the River Bann also flows through the southwestern areas of the county. Slieve Donard at 849 m (2,785 ft), is the highest peak in the Mournes (and in Northern Ireland) and the county covers an area of nearly 2500 square km with a population of nearly 550,000. 

It takes its name from dún, the Irish word fort, which is a common root in Gaelic place names (such as Dundrum).  The fort in question was in the historic town of Downpatrick, originally known as Dún Lethglaise ("fort of the green side”).

More information on Co Down's history, geology and tourist attractions can be found though these links:

https://​discovernorthernireland.com/​destinations/county-down

https://www.britannica.com/​place/Down-former-county-​Northern-Ireland