The Wicklow Mountains form the largest continuous upland in Ireland and occupy the whole centre of the county. Lugnaquilla, at 925 metres, is the highest peak in the range and the highest mountain in Ireland outside County Kerry. The range sits on the Leinster batholith, the largest mass of intrusive granite in Ireland or Britain - 1,500 square kilometres of slow-cooled magma now rounded by ice and rain. Glaciation cut the long valleys here: Glendalough, Glenmacnass, Glen of Imaal, Glencree. The Sally Gap and the Wicklow Gap, at 498 and 478 metres, are the only road passes through the mountains under 600 metres, and they are still the highest road passes in the country. Wicklow Mountains National Park, established in 1991, covers 205 square kilometres - the largest national park in Ireland.

Saint Kevin founded the monastery at Glendalough in the late sixth century, after crossing the mountains from Hollywood via the Wicklow Gap. By the eighth century it was a settlement of 500 to 1,000 people, a centre of learning and pilgrimage important enough to be attacked repeatedly by local tribes and later by Norse raiders. The English burned it in 1398. The round tower still stands. Around a million visitors come every year. Saint Palladius - a bishop predating Patrick - had landed at Wicklow harbour in 431 AD and founded three churches in the county before Patrick did the same in Armagh. The Christianisation of Ireland may have started here, in this corner of Leinster, before the saint most associated with it got going.

After the Norman invasion in 1170, two Gaelic clans were pushed out of Kildare and into the mountains - the O'Byrnes in the east, the O'Tooles in the west - and from their mountain strongholds both conducted a four-century campaign of harassment against the English. The Wicklow Mountains became known as the terra guerre, the 'land of war,' as opposed to the terra pacis, the land of peace, of the settled lowlands. At Glenmalure in 1580, Fiach McHugh O'Byrne defeated an English army badly. Twelve years later, Red Hugh O'Donnell escaped from Dublin Castle in winter and crossed the snow-blanketed mountains to reach Glenmalure. His companion Art O'Neill died of exposure on the journey. Red Hugh lost several toes to frostbite. The Wicklow Military Road, built between 1800 and 1809 to give British troops fast access to the rebel-haunted glens, runs the length of the range from Rathfarnham south to Aghavannagh - too late, in the end, to matter. The rebels had already faded into the hills.

Wicklow is nicknamed the Garden of Ireland, and for most of the 20th century it had the highest percentage of woodland of any county - 18.5 percent today, the second-highest in Ireland. Powerscourt Waterfall, at 121 metres, is the second-tallest waterfall in the country; the granite cliff it spills over was carved by glacial ice at the contact point between granite and mica-schist. Lough Tay, far below in its dark valley, is locally called the Guinness Lake because it sits on the Guinness family's estate and because its black water topped by white sand looks remarkably like a poured stout. The largest artificial lake in Ireland is here too - Poulaphouca Reservoir, 2,226 hectares created in 1940 when the Liffey was dammed, drowning the village of Ballinahown and relocating its 70 families. In droughts the old buildings still surface.

On 24 August 1986, during Hurricane Charley, 280 millimetres of rain fell on Kippure mountain on the Wicklow-Dublin border in a single 24-hour period. It remains the heaviest daily rainfall ever recorded in Ireland. A thousand homes had to be evacuated in Bray alone. The mountains regularly see fifty or more days of snow each winter - the snowiest part of Ireland - and easterly winter winds occasionally bring the kind of weather that has names: the 2018 'Beast from the East,' the Big Snow of 1947. Wolves were native here until 1710, when the last one in Wicklow was killed at Glendalough. Red deer were hunted to extinction and then reintroduced on the Powerscourt Estate in the 18th century. They have since interbred with Japanese sika deer that the same estate imported. Every deer you see in the Wicklow Mountains today traces back to the Powerscourt herd.