The name Inis Tuaisceart is descriptive and bureaucratic - Inishtooskert is merely a phonetic anglicisation of the Irish - but An Fear Marbh is the name with mood. The island sits at the northern end of the Blasket archipelago, slightly apart from its siblings, and when viewed from the east its long horizontal silhouette resolves into the unmistakable shape of a man laid out for burial. The illusion holds across light and weather, across centuries of looking. It is the kind of geographic coincidence that becomes folklore, and across the Dingle peninsula the Sleeping Giant has carried that nickname for as long as anyone alive can remember. The view from Dunquin and the headlands above Ceann Sleibhe is the classic one. From the air, the human shape collapses into a more prosaic outline - an island, treeless, north-south.

What looks like a corpse from the mainland is, in fact, exuberantly alive. Inishtooskert holds one of the most important seabird colonies in northwest Europe. The Seabird 2000 survey - a coordinated census run across Britain and Ireland at the turn of the millennium - counted more than 27,000 pairs of European storm-petrels on the island. That is the largest colony of the species anywhere in Ireland. Storm-petrels are tiny birds - smaller than a starling - that spend their lives over open ocean and come ashore only to breed, only at night, only to slip into crevices and burrows where the gulls and skuas cannot find them. To stand on the island in summer darkness is to hear them returning: a thin, churring call, like a sewing machine running underground.

Among the island's other inhabitants, long since departed, were early Christian hermits. Inishtooskert preserves an unusual concentration of ecclesiastical remains: a dry-stone oratory, beehive cells known as clochans, slab crosses, and the trace foundations of a small monastic settlement. These places were typically built between the sixth and ninth centuries, when Irish monasticism reached its most ascetic extreme and small communities of monks sought out the loneliest rocks in the Atlantic. Skellig Michael is the most famous of these refuges, but Inishtooskert is part of the same impulse. The stones survive because no one has been here to take them apart for newer purposes. They sit, slumped and lichened, where their builders abandoned them roughly a thousand years ago.

In 1970, David Lean - the British director of Lawrence of Arabia and Doctor Zhivago - released Ryan's Daughter, a sweeping romance set on a fictional version of the Dingle Peninsula in the year of the 1916 Easter Rising. The film won two Academy Awards and earned eight more nominations, and it brought tourists to West Kerry in numbers the region had never seen. Inishtooskert appears repeatedly in its long establishing shots. Lean's camera lingered on the island's recumbent profile against a turning sky, and the Sleeping Giant became, for a brief cinematic moment, a global silhouette. The film also helped reshape the local economy: hotels expanded, roads improved, and the formerly remote villages of the Gaeltacht acquired a new kind of fame.

Today there is no ferry service to Inishtooskert. Visiting requires a private boat, calm seas, and a tolerance for landings that are not really landings - the island has no harbour, just rocks where a rigid inflatable might pause long enough to disembark a careful passenger. Most people see the Sleeping Giant only from the mainland or from the deck of a Blasket ferry on its way to Great Blasket. Which is perhaps the right relationship with this place. The island is best appreciated as a presence, a fixed feature of the western horizon, the longest northern shape in the cluster, a man at rest above the world's largest assembly of storm-petrels. The wind comes off the Atlantic and finds nothing tall enough on Inishtooskert to break against. The grass leans east. The birds, when they return at night, can be heard before they are seen.