The highest points on the peninsula, Gainne Mhor and Gainne Bheag - anglicised as Ganiamore - carry the name of Grainne, the beautiful princess promised to Fionn but in love with Diarmuid. Legend places the fugitive lovers here on their flight to Scotland, resting on the hill before sailing onward into exile. Across the peninsula, the long beach of Tra Mhor, also called Tramore or Rosapenna Strand, is where Deirdre and Naoise are said to have camped while fleeing Conchobar mac Nessa, king of Ulster, and his Knights of the Red Branch. Two of the great star-crossed couples of Irish mythology supposedly passed nights on this small headland. Then in the late 4th century, history thickens into a different kind of legend: Conall Gulban, son of Niall of the Nine Hostages and the man for whom Tir Conaill - Donegal itself - is named, chased the warriors of Ulaid through Rosguill on his way to the shores of Lough Foyle. He and his brother Eogan became the ancestors of the kings of Tyrconnell and Tyrone.

Like much of Donegal, Rosguill belongs to St Colm Cille - Columba in English. Born at Gartan a few miles inland, a great-grandson of Conall Gulban himself, Colm Cille founded the monasteries of Derry and Kells before sailing into exile in Dal Riata. Along the way, legend says, he established smaller churches everywhere he went. The Old Church at Mevagh, in Clontallagh townland, is supposed to be one of them. The high cross in the churchyard supposedly carries the prints of his fingers, pressed into the stone. He spread curses and blessings in proportion to the hospitality he received. The townland of Dundoan Upper, where the locals once mocked him, was cursed to always have a fool in its community. Dooey and Island Roy, by contrast, were blessed: no one from either townland, the saint prophesied, would ever drown. Down at Downings, his footprints are said to be visible on a rock.

Rosguill organizes itself in the old Donegal way. The western side of the peninsula, looking out at Sheephaven Bay, is called The Bottom. The eastern side, on Mulroy Bay, is The Far Side. The townlands on the mainland end of the peninsula are The Upper Part. Nineteen townlands make up the peninsula itself - Melmore at the tip, then Gortnalughoge, Dundoan Lower and Upper, Glenoory, Dooey, Glebe, Clontallagh, Ardbane, and the others. Another thirty-one townlands extend the wider parish of Rosguill, also called Mevagh. Each name is a record of who once lived there, what grew there, what was killed there. Crop rotation in 1800 ran potato, barley, oats, flax - the staples that fed and clothed a tenant population whose lives were precarious in ways the modern visitor can barely imagine. Captain Nicholas Pynnar surveyed this land in 1618 to verify that the Planters were holding up their end of the bargain after the Plantation of Ulster.

Today, Downings Harbour on Sheephaven Bay has become an improbable centre for big-game fishing. Each autumn, charter boats leave the small Donegal pier in pursuit of giant bluefin tuna, fish that can run over 200 kilograms, drawn into the waters off the north coast by the same Atlantic currents that have always shaped this peninsula. Angling, hiking, golf, watersports - the modern offering is real, and the area attracts artists and musicians who find the light and the Irish-language community reason enough to stay. Rosguill carries scars too. Unchecked ribbon development through the 1990s pockmarked stretches of the coast, the consequence of weak planning enforcement and the same gold rush that disfigured so much of the Irish west. But away from the main settlements, one can still walk into landscapes that look much as they did when Colm Cille was supposedly leaving fingerprints in stone - heath and bog and pasture, the Atlantic crashing on the same rocks where Fionn supposedly caught up with the slayer of his father.