THE DAY IS COLD and bright as I walk up the quiet road through small wet fields edged with hawthorn and gorse bushes. The sky is ice blue; behind me, the road slopes down to the bay, whose green water moves with the quality of mercury, slow and heavy. It is early February in the west of Ireland, and there is the faintest warmth on the sea breeze coming over the bogs. But this is only a lull: winter storms will soon come back with fury.

I turn down the bóithrín (country lane) and hop an old drystone wall into scrubby hazel trees. Like most trees out here, they are stunted and windswept. But the muddy ground slopes down from the road, and as I head west, soon there are curving oaks covered in lichens, mossy granite boulders, and tangles of holly and bramble that scratch at my legs.

As the land falls away, the oaks get bigger, sheltered by the deepening valley, and then there are tall pines and boulders the height of houses. Soon I can see dark water moving through trees. I arrive at the river, which flows down hard from the high bogs, black and frothing. I climb through thick heather and bracken to a bluff. All I can see are woods and river.

But this is a mere sleight of the land, an illusion created by standing deep in small woods. Despite its lush and green image, only 2 percent of Ireland is native woodland, and all of it is in small fragments, like this eighty-acre remnant.

In the millennia that followed the last ice age, a great wood covered Ireland. The country’s folklore is populated by mythical warriors like Fionn mac Cumhaill, who was raised in secret in the forest of Sliabh Bladhma. His war tribe, the Fianna, roamed the great oak woods and were compared to a wolf pack.

But from the Stone Age on, waves of settlers cut, burned, and grazed the forests. The climate grew wetter too, and great bogs formed. The woods slowly withered and died. The last of Ireland’s great oak forests were gone by the end of the seventeenth century.

But pockets of this ancient forest linger: on coastal headlands, on old country estates—which never faced the same pressures as the land outside—and in remote valleys, like the one I’m standing in now. These are among the last remnants of temperate rainforest in Europe.

Throughout the year, the Gulf Stream brings warm, moist air from the Caribbean up Ireland’s west coast, which would otherwise be as cold in winter as Labrador. The humid, oceanic woods of western Ireland and Scotland are some of the richest habitats in the world for mosses and liverworts, rivaling the cloud forests of the Andes.

I find one of these species, the tender liverwort Plagiochila spinulosa, growing thickly on a boulder of pink granite. I crouch down with my hand lens: Plagiochila is simple and fragile, its spiny yellow-green leaves just one cell thick. All around me, polypody ferns sprout from trees, and bright green lungwort lichens flake off hazel trunks. On the dark bellies of streamside boulders, tiny filmy ferns, thin and translucent, tremble and shine like slices of green glass. The whole wood pulses with water. Beads of moisture cling to mosses like clear jelly from heavy rain yesterday. New pools and streams form a forest delta, and the wood echoes with the sound of dripping, fills with the scent of wet humus.

If I walk just deep enough into the wood so that I cannot see the light from its edge—but not so far that I start to come out the other side—I can imagine I am standing in an ancient forest wilderness. Maybe a lynx watches me from the undergrowth, or a pregnant she-wolf from a cavern beneath the next boulder. In some old Irish stories, wolves that denned in caves were seen as entering and exiting the otherworld.

But there are no wolves left here; the last packs were hunted to extinction in the eighteenth century. Brown bears were driven to extinction over two thousand years ago. Wild boar and lynx are gone too, as are great auks, capercaillies, bitterns, ospreys, woodlarks, and at least 115 other species this country has lost.

“Here on the island of Ireland, we live in ghost-land,” the Irish environmental journalist Ella McSweeney said recently, “marked not by what is around to see and hear, but what is not.” She was speaking at a debate on rewilding, on the potential to restore Ireland’s lost wilderness, and with it some of its former wild inhabitants. But this feels a far-off vision, given that so many of our surviving species (curlews, corncrakes) and habitats (raised bogs, wildflower-rich meadows) now cling to the precipice of extinction too, driven by a mix of agricultural intensification, sprawling development, intensive forestry, and peat extraction. The once relatively rich habitats that slowly replaced the great woods over millennia—large stretches of mountain heath, blanket bog, and pasture—are now fading too. So, Ireland’s ghost-lands grow.

A shower passes over the wood. The sky darkens to watery gray, and a cold gust moves through the trees. A storm is forecast, and I must get going. As I walk back down the bóithrín toward the bay, I look out over the valley, where this wood has been hiding out for the past ten thousand years. From down in its hollow, the russet of winter oaks blending into the red bog above, it is in fact strangely hard to see there is wood there at all.