From early morning, Rathlin O'Birne, the island hermitage, founded by Assicus (or Tassach), the favourite disciple of St Patrick and known to have given the saint the last rites, acts as a central focus for this intending explorer.

As the skies change hourly above the island, clear to cloudy to completely overcast, at times invisible, it is obvious that a crossing cannot be taken for granted. The sea alternates between angry and a deceptive calm concealing the rocks which make approaching Rathlin O'Birne, off the south-west coast of Co Donegal, so hazardous. Experienced boatmen persist in advising caution. Trawlers have gone down near here over the years. Many of the island's treacherous surrounding reefs have long since been mapped, recognised for centuries as dangerous.

Although formerly a place of pilgrimage, and possibly home to people living there as late as the 19th century, few casual visitors now venture out to Rathlin O'Birne. Lying about three nautical miles off Malin Beg, just to the south of Glencolmcille, on a clear day you can watch the sheep grazing on it. Nearly always visible is the lighthouse built in 1856. But as each day passes, the chances of sailing out to it remain poor.

"Are we going to the island today?" becomes a chant. It seems as if young James Ramsay had stepped out of the pages of Virginia Woolf's To The Lighthouse. Hopeful, anxious, despairing, determined, I suspect I am a nuisance.

It is not only the crossing and undercurrent that present difficulties, it is the landing as the sea rises and drops dramatically, hoisting a boat with it. Too wet, too windy, too rough, too hazy. Every gull coming in to land appears to be heralding a coming storm. Conditions aren't likely to be ideal, or even adequate. After almost a week of waiting, most locals agree it is not going to be possible this time, particularly as a day of apparently perfect conditions had not been considered sufficiently safe by the boatmen.

A sense of failure far worse than mere frustration takes over. Everything has been packed into the car, except for wellingtons and various maps, when a local farmer, Conal McGinley whose sheep graze on the island from about April to late October each year, reckons this hardy crypto-pilgrim is better able than most to jump to land. Even with the sky as grey as the sea - itself too choppy to ignore - the trip is exciting. Small boats always are. You feel more courageous, as well as vulnerable. Rising and falling through the water, the little boat, powered by its new 15-horsepower engine, carries on towards the stern cliffs. As the mainland is left behind, Malin Beg Martello Tower looks more imposing from the sea.

What was on the mind of Assicus the Gaul as he faced this place almost 1,500 years ago? Relief? Apprehension? Privilege? Fear? Did he smile at the seals? Was it sunny, or wet like today? As there are no traces of prehistoric habitation, the island was probably deserted when he established his hermitage here in 500 AD after the death of Patrick. The pinkish granite cliff-face is sheer. On reaching the top via steps constructed by the lighthouse service, the sea at the distant base of the steep, narrow gorge is dark blue.

It is a cove suitable only for either the toughest or the most desperate of pirates. A few gulls are already circling, unhappy about the presence of an intruder. The rain drives down. Walking across the island, away from the lighthouse, towards a knoll on the sheltered slope close to the north-east shore where the hermitage remains contained within a disintegrating dry-stone wall enclosure, the gulls become hysterical.

The rain is getting heavier, a frail yellow light illuminates the sky. The roar of the sea is upstaged by the relentless shrieks of increasingly irritated birds. Visiting here in May during nesting carries the risk of being attacked by protective parent birds. It's quieter now. But will the camera survive all this dampness?

Assicus, who was a skilled metalworker, built his small cell here. Long since collapsed, it is built in a non-native style, reminding us that he is believed to have come from Gaul. Its entrance faces the west doorway of an oratory in which Schist headstones mark the graves. Each is inscribed with a simple Latin cross, now outlined with red paint. A large font on the tomb-shrine is full of water believed to possess healing properties.

Outside this enclosure is the rectangular base of a leacht, with a small Schist slab. The presence of other crosses and cross slabs indicates a turas or pilgrimage route - but one not performed in living memory. Beneath this slope is a beautifully constructed well. A few feathers float on the water.

Kneeing in under its roof, I shelter to reload the camera. Rain is streaming down my face, the wax has washed off my coat. The mossy ground is pleasant to run on even in big boots, until one foot sinks abruptly into a deep hole recently vacated by a nesting puffin.

It is difficult to recall ever being quite so wet when fully dressed, particularly while wearing a bulky climbing jacket and long - if no longer waxed - riding coat. But only the camera matters; I'll dry.

There is a large rectangular church building and more enclosures. Also evidence of terraces; the hermit and later inhabitants must have planted food. Had penitents once stood in the small pond? Or had baptisms taken place there? The extent of the building suggests that others followed Assicus, either as hermits or as members of a small community. But it is the hermit's experience which intrigues. Was he often hungry? Had he seen great sea monsters? Did the Devil tempt him?

Far from the romance of a Pacific island, an Atlantic hermit must contend with the harshest of winter storms. Was Assicus terrified by the massive waves flooding his retreat? Waves powerful enough to have smashed part of the mighty stone wall built by the lighthouse service.

Shrouding the camera with my coat, I film the birds. Nothing rare. Mainly herring gulls, some fulmars. Three depressed-looking cormorants stand in a miserable line. The gulls favour more defiant poses, gazing out over the sea, flying off as the wet-looking intruder approaches.

Tramping over several individual beaches of beautiful rounded stones, with occasional mounds of rotting seagull carcasses, I think about Assicus; his sanctity, his solitude, his thoughts as he gazed at the Donegal coast. Overcome not by prayer, but by the desire for chocolate. Or bread. Even fish. No, not fish - the stench of the seaweed is too pungent, I'm suddenly incredibly hungry. The farmer reappears. None of the sheep has died. But one lamb is injured. A rear leg has been cut to the bone by discarded wire.

The boat has returned. Time to leave. It is getting darker. The Scotch Black Face is as passive as a sleepy Collie, but at five months is a bit bigger. The return trip seems shorter, undramatic. Hugging the sheep, I admire the stones I am bringing back and recall reading that the aged Assicus, when forcibly taken from another island hermitage, wept.