On a Wednesday after St. Brendan's Day - the saint's feast falls on 16 May - in 1527, Manus Ó Domhnaill began building the castle of Leithbher. He finished the masonry and woodwork by the end of that same summer, even though the O'Neills of Tír Eoghain were at war with him. That kind of speed tells you something about Gaelic Ireland: stone and oak rising in a season, in defiance of an enemy on the next hill. The castle changed hands more than once. In 1543 it was handed to a member of the O'Gallagher clan to guard for the O'Donnells, and the O'Gallaghers promptly tried to keep it for themselves. Calvagh O'Donnell brought English troops and 'ordnance and engines for taking towns' the following year to wrest it back. A late sixteenth-century bardic poem still survives - 'A beloved dwelling is the castle of Lifford, homestead of a wealth abounding encampment; forge of hospitality for the men of Ulster, a dwelling it is hard to leave.' The castle is gone now. Only the poem remains.

Lifford's centre is The Diamond - an Ulster Plantation pattern of crossroads and open ground that you'll see repeated in Donegal Town and Derry. The handsome Lifford Courthouse, built in 1746 to designs by Michael Priestley of Dublin, anchors The Diamond and is now a heritage centre and restaurant. Walk through it and you can see original holding cells in the basement and a display of O'Donnell clan documents and artefacts. Across the way is the County House, the headquarters of Donegal County Council, which still administers Donegal from this seat at the very edge of the county. The Battle of Lifford was fought in 1600 during Tyrone's Rebellion. After O'Doherty's Rebellion was defeated at Kilmacrennan in 1608, captured rebels were brought to Lifford to be tried and executed - the most famous being Phelim Reagh MacDavitt.

Three miles outside Lifford, just off the N14, sits Cavanacor House - one of those quiet seventeenth-century buildings whose unassuming exterior conceals a strange connection. Magdelene Tasker was born there in 1634. She would marry Captain Robert Bruce Pollock, emigrate to the United States, and become the great-great-great-grandmother of James Knox Polk, the eleventh President of the United States. There is more: in 1689, on his way to lay siege to Derry, King James II and VII dined at Cavanacor House. Two centuries of American and British history pass through one dining room in Donegal. The estate later became an Irish Defence Forces military barracks; the barracks closed on 31 January 2009. The house is still there, and the long American thread runs back through it to a baby born during the year of Charles I's coronation.

Drive from Dublin or Belfast and Lifford is the first town in Donegal you reach. The N2 from Dublin and the A5 from Tyrone funnel into one place - cross the Lifford Bridge and you are in the Republic. The town has long called itself 'the Gateway to Donegal,' and the description still holds. Beyond Lifford the road branches: the N15 to Sligo via Donegal Town, the N14 to Letterkenny, the R236 to Derry. Bus Éireann routes to Dublin Airport and Dublin City start here. The greyhound stadium - one of the most famous in Ireland - hosts races throughout the year. The town also gave Ireland the goalkeeper Shay Given, who held the Republic's caps record for years until Robbie Keane overtook him, and the Eurovision singer Mickey Joe Harte. For a place that has been hosting through traffic for four centuries, Lifford is remarkably easy to drive past without realising what is here.