Before any Hamilton crossed the Irish Sea, this stretch of forest and water belonged to a senior branch of the Ó Néill dynasty - the Gaelic kings of Tír Eoghain. One of their towerhouses sat on Island McHugh, a crannóg in what is now called Lough Catherine, just north of the modern house. After the Nine Years' War broke the Gaelic order, the Crown confiscated these lands. Around 1610, Sir George Hamilton - a Catholic, brother of the 1st Earl of Abercorn, from Renfrewshire - was given a parcel of confiscated Ó Néill territory in the Barony of Strabane Lower. He built Derrywoone Castle as his seat, a Lowland Scots-style bawn whose ruins still stand. The crannóg on the lough, the bawn in the woods, and the Georgian mansion that came later all sit within a few miles of each other - three centuries of architecture in one parkland.

The current Baronscourt was designed by George Steuart in 1778 and built over the next four years - seven bays wide, three storeys high, with coupled Tuscan columns and a rotunda at the centre. The Abercorn coat of arms was carved into the pediment. Then John Soane, the architect who would soon become famous for the Bank of England, was hired to remodel it in 1791. He flipped the front and the back. He combined the drawing room, parlour and hall as far back as the pillars into one continuous gallery. He designed a glasshouse. It was bold, and it lasted exactly five years before a fire in 1796 destroyed everything but the wings, and the whole house had to be rebuilt by Robert Woodgate from 1797 to 1798. The Morrisons came in for further work in the 1830s. David Hicks remodelled the library in the late twentieth century. Few houses in Ireland have been more continuously fussed over.

On 17 May 1918, with the Irish War of Independence approaching, Baronscourt became a target. The 3rd Duke was chairman of the Ulster Unionist Council and had urged Loyalist forces to 'die if necessary in opposing home rule'; the house was widely understood to be a depot for Ulster Volunteer arms. The Strabane Chronicle described 'a most exciting raid for arms by a very large force of masked and armed men in the early hours.' The IRA volunteers cut the telephone and telegraph lines, detained the landsteward Robert Bell and removed the night watchman Edward Young four miles into the countryside, then methodically searched the house. The female servants reported that they were treated with complete courtesy. What the raiders took remains contested - Unionist papers said only six 'rusty swords' and portraits of the King and Queen; Republican papers claimed 'a large quantity of arms and ammunition.' Estimates of the raiding party ranged from sixty men to two hundred, in anywhere from eight to twenty cars. No one was ever arrested for it.

Baronscourt today is a working estate. The neo-classical mansion, Grade A listed, sits in an Italianate parkland of formal gardens and woodland that runs down to the loughs and the wider demesne. The 18-hole golf course celebrated its centenary in 2014. The traditional burial place of the Dukes - the 1st, 2nd, and 3rd Dukes, and Sacha, Duchess of Abercorn - is the graveyard at Baronscourt Parish Church. The current Duke still uses the house as his Irish residence; this is, in the old word, the caput, the chief seat, of the Abercorns in Ireland. Drive west from Newtownstewart on a sunny morning and you will see only stone walls and oak trees - but behind them is one of Ulster's longest-running stories of land, power, and the slow churn of who owns what.