The A21 is a road in County Down

County Down

-Cities:*Belfast *Newry -Large towns:*Dundonald*Newtownards*Bangor-Medium towns:...

 in Northern Ireland

Northern Ireland

Northern Ireland is one of the four countries of the United Kingdom. Situated in the north-east of the island of Ireland, it shares a border with the Republic of Ireland to the south and west...

. The route commences in Bangor

Bangor, County Down

Bangor is a large town in County Down, Northern Ireland. It is a seaside resort on the southern side of Belfast Lough and within the Belfast Metropolitan Area. Bangor Marina is one of the largest in Ireland, and holds Blue Flag status...

, passing through Newtownards

Newtownards

Newtownards is a large town in County Down, Northern Ireland. It lies at the most northern tip of Strangford Lough, 10 miles east of Belfast, on the Ards Peninsula. Newtownards is the largest town in the Borough of Ards. According to the 2001 Census, it has a population of 27,821 people in...

, Comber

Comber

Comber is a small town in County Down, Northern Ireland. It lies 5 miles south of Newtownards, at the northern end of Strangford Lough. It had a population of 8,933 people in the 2001 Census. Comber is part of the Borough of Ards...

, and Ballygowan

Ballygowan

Ballygowan is a village and townland in County Down, Northern Ireland. It is within the Borough of Ards. The town of Comber is a short distance to the north-east, the town of Saintfield to the south, and the city of Belfast a further distance to the north-west. It had a population of 2,671 people...

, and finishes on the northern outskirts of Ballynahinch.

There are two dual carriageway sections; between Bangor and Newtownards, and between Newtownards and Comber. The second of these passes near the northern shores of Strangford Lough

Strangford Lough

Strangford Lough, sometimes Strangford Loch, is a large sea loch or inlet in County Down, Northern Ireland. It is separated from the Irish Sea by the Ards Peninsula. The name Strangford is derived ; describing the fast-flowing narrows at its mouth...

 through some of the best agricultural land in Ireland in the vicinity of Comber. This section also passes within 2 miles of Scrabo Tower

Scrabo Tower

Scrabo Tower is located to the west of Newtownards in County Down, Northern Ireland.The landmark, which is visible from most of north Down, was built on a volcanic plug above the town in 1857 as a memorial to Charles Stewart, 3rd Marquess of Londonderry who was one of the Duke of Wellington's...

.