26 August 2016 Last updated at 10:29 BST
An archaeological excavation in Clones, County Monaghan, had failed to uncover the castle but, unknown to experts, it was standing nearby.
County heritage officer Shirley Clerkin took BBC News NI's Julian Fowler on a tour.
A 17th century castle "lost" for more than 250 years has been rediscovered in the centre of a town on the Irish border.