Active Autumn: This lough marks the spot for hidden wildlife treasure

Anywhere but Northern Ireland and this place would be overrun by tourists, says Mark Rowe

The oysters, according to my colleagues, were delicious, served with lemon and black pepper, and I could vouch for the giant fish pie as we gazed at the gannets and Arctic terns diving into the almost still waters.

It was just another dreamy day in Northern Ireland's unexpectedly idyllic rural hinterland.

Those oysters were served on the terrace at Daft Eddy's, a pub on Sketrick Island, by the western shore of Strangford Lough. They'd made the short journey from the Cuan Oysters farm next door, where 400 tons of them are grown on the bed of the lough every year. Another neighbour is the teetering ruin of 15th-century Sketrick castle, brought to its knees by a storm in 1896 and now mantled in ivy and lichens. It was all impossibly scenic.

Strangford Lough is a mightily impressive stretch of water, which, in any other part of the country, would be overrun by visitors. The lough begins just south of Belfast and runs for an extraordinary 20 miles due south, where it pours through fast-flowing narrows (so fast that a tidal power scheme has been built here) into the Irish Sea. More than 100 islands and islets are dotted around the lough, some substantial, marooned hillocks, while others, flat and low-lying, known as pladdies, seem almost ephemeral. Cormorants line up along sand or gravel banks exposed only at low tide, drying their feathers in the light winds. Now and then a minke whale or a basking shark finds its way into the lough.

The best way to explore is on foot or by bicycle, following the winding narrow lanes on its western shore. There's little traffic and much to see. Navigate around a bend and you could surprise a heron poised to pounce on unwary fry in the seaweed. Tiny creeks squeeze into land and are wonderful places to pause and explore.

You'll soon come to Nendrum Monastery, located on one of the lough's islands and dating from the fifth century. It enjoys a remarkable elevated setting, encircled by trees and comprising three concentric rings. Early Christian materials were found here, including brooches and dress fasteners, along with a graveyard. Just below the monastery, low tide reveals the remains of a seventh-century tidal mill.

There are reckoned to be 140 shipwrecks at the bottom of the lough, and divers, seemingly happy to pick their way through centuries of goose poo, pull up the occasional medieval treasure or curiosity. The lough has been worked for centuries by man, though perhaps its most surreal moment came in the 1970s when it was used as a base for a runway and the Bay City Rollers were among those to arrive by air. Further afield lies the picturesque village of Strangford, while you'll find Cistercian remains in the village of Grey Abbey, along with a clutch of antique shops.

The best way to get a handle on the natural riches of the lough – it is the most important wildlife site in Ireland – is to visit Castle Espie, a wildlife reserve managed by the Wildfowl and Wetlands Trust for the past 20 years. A recent £3m restoration project has enhanced freshwater and inter-tidal habitats with the aim of attracting yet more marine and birdlife. There's a small exhibition recalling how the lough was briefly used as a centre for quarrying and brick making, and children will adore Stoat Town, a woodland play area with equipment made from trunks and branches. Among the 40 species of resident birds are the threatened white-headed duck and the whistling or tree duck.

Usually, it's the grounds of such places that are the eye-opener, but in the case of Castle Espie, the visitor centre is worth nosing around, too. It has been voted Northern Ireland's sustainable building of the year and embraces "slow" architecture, sourcing local materials and installing compost toilets, a biomass boiler, rainwater recycling and solar panels.

The jewel in the reserve's crown is the autumn arrival of squadrons of Brent geese from the Canadian Arctic. The spectacle of these birds, up to 35,000 of them, feeding on the tide line is one of the UK's great wildlife experiences. Elsewhere, as you stroll around the grounds and shoreline paths of the 60-acre site, you'll come across idyllic ponds and woodlands, observation hides, and even a reconstruction of a crannog, or traditional Irish dwelling, home to swallow nests.

"This landscape is so important," said Kerry Mackie, Castle Espie's biodiversity ranger, whose father was involved with conservation around the lough in the decades before WWT took over. "We're at the end of the flyway for many birds. You have diverse habitats, and you can get very close to the wading birds. If people like my father and the National Trust had not got involved, then God knows what would have happened. The geese were getting clobbered [by wildfowlers] and it was pretty much a free for all."

Generally, you read about wetlands only in the context of their devastation. Castle Espie could easily have been ruined by developers. But, for once, this is a wildlife story and experience that takes a more optimistic turn.

Compact Facts

How to get there

Mark Rowe stayed at Rayanne House (028-9042 5859; rayannehouse.com), which offers double rooms from £90 per night.

Further information

Strangford Lough (discovernorthern ireland.com); Castle Espie (wwt.org.uk/visit-us/castle-espie).