It was not a simple question to answer. I’d driven up and down the gorgeous green valleys of the Mountains of Mourne on my last visit to Northern Ireland a couple of years ago. But I couldn’t honestly say I’d actually seen them, because the peaks of County Down’s celebrated coastal mountains had been hidden under mist. One day, I’d promised myself then, I’ll come back and walk the high paths of the Mountains of Mourne.

I was pretty certain that the Mournes would be stiff with walkers, world-famous as they are through Percy French’s tacky but touching old song. But I couldn’t have been more mistaken. If the Mournes were magically transplanted across the Irish Sea, they would be as oversubscribed by ramblers as the Lake District fells. But the high places of the Mountains of Mourne are terra incognita to almost everyone. Millions have heard of them. Tens of thousands come to view them each year. But only a handful of hill walkers actually get to know them.

Astonishingly, there’s almost nothing available to guide the outsider among these beautiful, easily walkable mountains. For experienced map readers the 1:25,000 “Mourne Country Outdoor Pursuits” sheet will open up the whole area like a key in the lock of a treasure chest. But no walker-friendly books or leaflets dedicated to Mourne walks are available in Newcastle, the Mournes’ gateway town, either at the Tourist Information Centre or the Mourne Heritage Centre. There used to be an excellent Mourne Mountains Walks pack of 10 walks, but it is out of print. The local tourism authorities are surely missing a vital trick here.

It’s an ill wind, was my selfish thought as James McEvoy and I set off up the Trassey Track on an uncharacteristically glorious morning of cloudless blue sky over County Down. The only other walkers we met all day were up among the peaks, and there was only a sprinkling of these. On such a day in the Lakes it would have been heel-to-toe single-file crawling along the hill paths. The sense of elbow room and empty skylines in the Mournes was intoxicating.

James has put in 14 years as a ranger in the Mournes. Next year he’ll be celebrating his silver jubilee as a member of the Mourne Mountain Rescue team. You couldn’t ask for a better man to show you what you’d otherwise miss. “See these slots in the rock here?” James said, stooping to insert his finger into some semi-cylindrical grooves. “That’s where the quarrymen’d work with the plug-and-feathers method, splitting the stone along the invisible joints that they could tell were there.

The Mournes are all granite, you see, and that was a big quarrying industry back in the 19th century. The stone would be dressed here and shipped away in granite schooners to Belfast, across to Liverpool and the northern cotton towns. In fact, there’s a saying that the Mournes paved Lancashire.”

The old quarry road of the Trassey Track climbed gently towards Hare’s Gap, a high saddle between rounded Slievenaglogh and the jagged granite tors on Slieve Bearnagh. Up there we stopped to gaze back down the Trassey Valley, then sat to eat our sandwiches in the shelter of the Mourne Wall.

The Wall is a shoulder-high ribbon of stone wall that circles for 22 miles through the Mournes, running up and over 12 of the highest peaks. It was built as a boundary marker early in the 20th century by the company that created the reservoirs of Silent Valley and Ben Crom in the heart of the mountains; and it stands as a monument to the hundreds of local men who dressed its stones to earn their bread in hard times.

It wasn’t only granite quarrymen and the builders of the Mourne Wall who travelled these mountain paths. Back in the 18th century, when brandy and tobacco made good profits for anyone bold enough to dodge the excisemen, the Mournes provided a quick and secure through-route for contraband goods from the Down coast into the heart of Ireland. The path that carried James and me from Hare’s Gap eastward under the peaks of Slieve Corragh and Slieve Commedagh is known as the Brandy Pad, an old smugglers’ track from which we got superb views down over the darkly gleaming Ben Crom reservoir.

We stopped to drink cold water from a cleft cut by a mountain tributary of the Kilkeel River, then went on along the Brandy Pad. All around us the peaks of Mourne rose in rounded green heads, the distant line of the Mourne Wall swooping up and down their napes. “We used to have an annual Mourne Wall walk,” James reminisced. “One year a couple of lads brought bagpipes with them and they set off up Slievenaglogh from Hare’s Gap, playing while they climbed.” He began chuckling quietly. “As the gradient increased you could hear the tune getting more and more wobbly and strange, till it faded out altogether in a great lot of spluttering.”

We caught up with the Wall again at The Saddle, another high col under the shoulder of Slieve Donard, and went skidding and stumbling down a rubbly track beside the Glen River, with a wonderful prospect ahead over Newcastle and the great sandbank-streaked arc of Dundrum Bay. We passed the old domed ice house of Donard Lodge and descended through the woods around the Glen River’s miniature gorge to arrive in a sunbaked Newcastle.

Later that night, honking my harmonicas alongside the musicians of the Murphy family in Quinn’s Bar, I found my cheeks burning each time I pursed my lips to play. Was it embarrassment at my lack of finesse, or the Mourne mountain sunshine, or just the Guinness? I couldn’t be sure – but whatever the cause, it felt just fine.

Travel EasyJet (www.easyjet.com) and FlyBe (08705 676676, www.flybe.com) fly to Belfast; Avis (0870 6060100, www.avis.com) rents cars at Belfast’s airports.

From Belfast take A7 to Downpatrick, A25/A2 to Newcastle. Newcastle to Meelmore Lodge car park (start of walk): July and August – Mourne Rambler bus runs hourly, 10am-5pm. Sept-June – Donard Taxis (02843 724100): £6. By car: B180 Hilltown road; in 4.5 miles, left (319323 – “Meelmore Lodge” sign) for 1.25 miles.

Walk directions From Meelmore Lodge car park (305307), “Mountain Walk” sign on wall points left. In 30yds go through gate and along walled lane. At end (310302) go through gate; bear left, aiming for gate and stile ahead. Cross stile; bear left along wall to turn right (313303) up stony Trassey Track for a mile. Where track zigzags up to right (319290), aim ahead on clear path, then through boulders, up to gap and ladder stile in Mourne Wall, where it crosses Hare’s Gap (323287).

Cross stile; bear left along Brandy Pad path and follow it under Slieve Commedagh for 1.333 miles to cairn on col between Commedagh and Slieve Beg (342279). Continue to east end of The Castles cliffs (on your left); bear left here (348279), uphill off Brandy Pad to cross Mourne Wall at The Saddle between Slieve Commedagh and Slieve Donard (350279).

Follow clear path NE down Glen River Valley. In 1.333 miles, pass ice house (364295) to reach dirt road by concrete bridge (366297). Left for 50yds, then right, down stony track through trees on left of Glen River, to humpbacked second bridge (370299). Turn right to cross it, then left (“Glen River Track” sign) to continue descent on right of river, to Donard Bridge (372302). Bear left to cross bridge; descend beside river to emerge into Donard Park (373304); continue down to car park in Newcastle (374306).

• Length of walk 6.5 miles – allow five hours.
• Conditions Many stretches tricky underfoot, especially descending from The Saddle into Newcastle.
• Wear hill-walking boots and take wet-weather gear.
• Refreshments None en route – take water and picnic.