Mountains in County Down, Northern Ireland: The Mournes

County Down has no mountains entirely within its borders — but the Mourne Mountains straddle the Down/Armagh boundary and dominate the county's southern horizon.

The Mourne Mountains (Na Beanna Boirche)

• Highest peak: Slieve Donard — 850 m (2,789 ft) — highest in NI & Ulster.
• Other major summits: Slieve Commedagh (767 m), Slieve Binnian (747 m), Slieve Bearnagh (739 m), Slieve Meelmore (680 m), Slieve Meelbeg (708 m).
• Geology: Granite pluton (50 Ma) — distinctive tors, steep gullies.
• Mourne Wall: 35 km dry-stone wall (1904–1922) enclosing 9,000 ac water catchment (Silent Valley, Ben Crom reservoirs) — Belfast's water supply.

Other Uplands in County Down

• Dromara Hills / Slieve Croob: 534 m — rolling drumlin country, not mountain terrain.
• Holywood Hills / Castlereagh Hills: Low basalt escarpments near Belfast.

Access & Recreation

• Gateway towns: Newcastle (coast), Castlewellan, Hilltown, Rostrevor.
• Hiking: Ulster Way sections, Mourne Wall challenge (22 peaks, 30 km), Slieve Donard via Glen River (most popular).
• Climbing: Granite trad routes (Hares Gap, Eagle Mountain, Pigeon Rock).
• Mountain biking: Rostrevor Forest trails (World Cup venue).

Cultural Touchstone

C.S. Lewis (born Belfast, holidayed in Rostrevor) said the Mournes inspired Narnia. Percy French's "Mountains of Mourne" — unofficial NI anthem.

So: No mountains in County Down proper, but the Mournes are County Down's soul.