Killymoon Castle was constructed in 1802-3 for Col James Stewart, to the designs of architect John Nash, who famously laid out Regent Street in London. It was his first castle in Ireland, and reputedly cost £80,000 to build. It has much in common with a later Nash castle of 1807-10 in Caerhays, Cornwall.
The celebrated gardener and designer of the Crystal Palace, Sir Joseph Paxton wrote: "I have visited most of the celebrated country seats in the Kingdom and a very large number on the continent, and I have never seen one - for the extent of it - more compact, more perfect in itself, or where the highest natural beauties have been more aided by refined taste and judgment, than Killymoon".
While much of the demesne was sold off after 1922, the house and surrounding planting survives. It has been described by the eminent architectural historian Terence Reeves-Smyth as ‘the oldest surviving example of a ‘castle style’ building by Nash and one of his most successful houses, boasting among other things, a very early full essay in the revived Norman style, a striking port cochère, and a revolutionary and ingeniously planned interior.’