The bedrock of Kilcommon parish is among the oldest exposed rock in Ireland. Pink striped pre-Cambrian gneisses here are roughly two billion years old. Pale creamy psammite and dark schists make up much of the rest, with snow-white quartz boulders that intruded from below during the Silurian period some 450 million years ago. When local farmers cut turf for fuel, they peel back the bog and expose this ancient bedrock, a window onto a landscape that has been heated, frozen, compressed, and tectonically rearranged across geological time. The country looks deceptively young - all bog, all wind, all green and brown. The stone beneath is older than complex life itself. Most of the things that have lived on this planet did so on this rock.

The blanket bog of Kilcommon shifts through the year - fresh green in spring, purple and gold with bog cotton in summer, the rustic orange-red of bog species lighting the landscape in November. It supports a fauna unlike anywhere else in Ireland. The corncrake, vanished from most of the country, still calls here. Grey-fronted geese pass through on the long journey north. And the red-necked phalarope - a delicate wading bird that breeds in the high Arctic - has its last remaining Irish breeding ground in this remote corner of Mayo. The phalarope is unusual in the bird world: the females are the brightly coloured ones, and they compete for males who do all the incubation and rearing. Their continued presence here is a thread connecting Mayo to Iceland and Greenland. When they leave, they will probably not come back.

The parish takes its name from Saint Coman, who lived at the end of the sixth century and is said to be buried in the old churchyard at Pollatomais near the entrance, where part of one gable wall of the old church still stands. In 1838 the Ordnance Survey writer John O'Donovan visited the site and noted that little could be learned from the ruin of its style or age. The saint, the church, and the parish name have outlasted the building. Five Catholic churches now serve the parish, including Seipeal Muire gan Smal at Ceathru Thaidhg, where Mass is celebrated in Irish. Between seven hundred and a thousand native Irish speakers still live in Kilcommon - a Gaeltacht that has weathered emigration, school closures, and the slow advance of English.

Benwee Head, on the parish's north coast, rises 255 metres (837 feet) straight out of the Atlantic. The sea cliffs run along the coastline from Benwee to Glinsk Mountain, broken by inlets and small harbours, with the rocky islands called the Stags of Broadhaven offshore. There are no roads to many of the most spectacular viewpoints. The walks have been marked and mapped by local groups - Comhar Dun Chaochain Teo and Siuloidi Iorrais - but they remain remote in every sense. From the top of Benwee on a clear day, the world drops away in all directions: the cliffs, the empty Atlantic, the small rocky islands turned white with breeding seabirds, the blanket bog rolling back inland under a vast sky. There are few places in Europe where the sense of being at the edge of the continent is so absolute.

Beginning in the early 2000s, Royal Dutch Shell proposed to run a high-pressure pipeline carrying raw unodourised gas from a field 85 kilometres offshore through Kilcommon parish to a refinery built ten kilometres inland. Local opposition was sustained and sometimes confrontational. After many years of disputed routes, oral hearings before An Bord Pleanala, and changes to the proposal, construction began in July 2011. In 2007, a Rossport farmer named Willie Corduff won the Goldman Environmental Prize for his role in the opposition. The Pipe, a documentary by Risteard O'Domhnaill, premiered in 2010 and became a touchstone of Irish environmental cinema. Whatever one thinks of the project, it asked a question that Erris had not been asked before: what is this landscape worth, and who decides?