Saint Seanach was the brother of Senan mac Geirrcinn, one of the better-known figures in the early Irish church, who founded the monastery on Inis Cathaigh - Scattery Island - at the mouth of the Shannon. The two brothers founded paired island monasteries at the western edge of their world, one in the Shannon estuary, one in Tralee Bay. The Illauntannig settlement is sometimes grouped, by archaeologists who study early Christian isolation, with the better-known Skellig Michael off the Iveragh Peninsula and Inishmurray off Sligo - all of them small, exposed, deliberately remote, the work of monks who took the desert tradition of the Egyptian Fathers and applied it to the cold Atlantic instead of hot sand.

The site that the Office of Public Works oversees today consists of two small oratories - one of them boat-shaped, an unusual form found at only a few early Irish sites - three clochan, the beehive-shaped drystone huts that monks used as cells, a souterrain with a wall-chamber, three leachta or stone altars, and a burial ground. A stone cross 1.82 metres tall stands beside the middle leacht. Its edges are bevelled. Holes drilled into the stone show that a plaque, long lost, was once mounted on it. Inside the main oratory, three cross-slabs lean against the wall. Outside, fragments of five quern-stones - hand mills for grinding grain - have been recovered. The entire complex is enclosed by a substantial cashel, a circular drystone wall, built without mortar. The use of curving walls and round huts has led archaeologists to cite Illauntannig as a textbook example of the curvilinear ecclesiastical enclosure - a deliberate aesthetic of early Irish monasticism.

The monks were not the first people on Illauntannig. A shell midden - a refuse heap of empty mussel, periwinkle, and limpet shells, the accumulated kitchen waste of generations - has been found inside the monastic enclosure. Radiocarbon dating places the midden in the late Neolithic or early Bronze Age, four thousand years or more before the founding of the monastery. Some archaeologists have argued, on the basis of the cashel's plan, that the monks did not build their enclosing wall from scratch but reused an existing ringfort, a defensive enclosure of much earlier date. The site, in other words, was a holy place or at least an occupied place long before Christianity reached this coast. The monks fitted themselves into a landscape that already had history.

Among the artefacts recovered from Illauntannig is a hand-bell of the type used to call monks to prayer, dated to between 600 and 900 AD, and an iron hammer. Both are now in the National Museum of Ireland in Dublin. One of the oratories has architectural features - in its altar and in the dressing of certain stones - that resemble the Irish Romanesque style, suggesting the site continued to be used and modified well into the twelfth century. Between the late nineteenth and early twentieth centuries, antiquarians and clergy made attempts to preserve the site against further collapse, recording the structures and stabilising what they could. The monastery's last working community is long lost to record. The stones, the bell, the hammer, the burial ground - these constitute the evidence of a continuous spiritual occupation that lasted, in some form, for more than seven hundred years.

Illauntannig is not easily reached. The Magharee Sound separates the Seven Hogs from the mainland; even on calm days the channel can be uncomfortable. Boats run from Scraggane Pier at the tip of the Maharees in summer, weather permitting, ferrying small groups of pilgrims, archaeologists, and the curious to the island. There is no scheduled service. The monastic enclosure has restricted access, protected as a National Monument. A working farmhouse - the only dwelling on the island - is rented out to summer visitors. To stand inside the cashel wall, surrounded by huts a monk slept in fifteen centuries ago, looking across the Sound to the mountains of the Dingle Peninsula, is to feel the particular Irish quality of religious geography: not just where you pray, but how far you had to go to get there.