This O’Donnell stronghold once commanded a vital ford crossing the River Finn, serving as both a defensive position and a statement of clan power in medieval Donegal. The castle gained particular prominence in 1601 when Niall Garbh O’Donnell, a controversial figure who sided with the English during the Nine Years’ War, made it his residence. Following the Flight of the Earls and the subsequent plantation of Ulster, the castle passed into English hands and underwent significant transformation under Sir John Kingsmill in the early 17th century.

Kingsmill’s improvements to the site were substantial; by 1622, he had “re-edified” the castle with significant additions and constructed an impressive bawn, a fortified enclosure typical of plantation architecture. Nicholas Pynnar’s detailed 1618/19 survey describes this bawn as a formidable structure: a square limestone fortification spanning 100 feet on each side, complete with two-storey flankers that provided both defensive positions and comfortable lodgings. At its heart stood a robust three-storey stone house where Kingsmill resided with his family, and nearby, a new village began to take shape around this plantation settlement.

The original medieval castle had fallen to ruin by the 1650s, but the story of the site continued to evolve. Local tradition maintains that Kingsmill’s bawn wasn’t built atop the O’Donnell castle ruins but rather in what became the centre of Castlefinn village, on grounds previously occupied by a manor house. Today, St. Mary’s Roman Catholic Church stands where this 17th-century fortification once dominated the landscape; the bawn site had been occupied by Captain Butler’s family home until its demolition in the mid-1960s, after which the church was constructed, adding another layer to this palimpsest of Irish history.