The castle witnessed centuries of drama, including a fierce family feud in 1434 when Niall O’Donnell besieged his brother Naghtan here, though the fortress proved too strong to capture. By 1480, this very castle became the site of reconciliation when the O’Donnell family held a peace conference, settling their disputes and confirming Egneghan O’Donnell as tanist of Tirconnell. The medieval stronghold continued to play a crucial role in local politics, with Niall Garbh O’Donnell taking residence here in 1601 during the tumultuous Nine Years’ War period.

Following the Crown plantation of Donegal in the early 17th century, Sir John Kingsmill transformed the medieval castle into a formidable plantation stronghold. By 1622, he had substantially refortified the structure, adding a strong bawn; a defensive wall enclosing the castle grounds. Contemporary records from 1630 paint a vivid picture of Kingsmill’s improvements: the stone house measured 45 feet long, 36 feet wide and 32½ feet high, whilst the bawn walls stood 12 feet tall and stretched 276 feet in circumference, complete with defensive flankers at each corner. The Crown clearly valued this strategic location, granting Kingsmill the power to hold weekly Monday markets and two annual fairs at Castlefinn in 1631.

The castle’s story ended not with siege or battle but with practical reuse; by the 1650s it had fallen to ruin, and around 1774 its stones found new purpose in constructing the first bridge across the River Finn. When Lieutenant Wilkinson surveyed the area for the Ordnance Survey in 1836, he found no trace of the once mighty fortress, noting that all the stones had been incorporated into the bridge. Today, whilst the physical castle has vanished, the area between the old railway station and the river still bears the name “Castle Garden”, and the village itself preserves the memory of Caislen-na-Finne, the castle on the Finn.