In 1753, the castle and lands passed to the fourth Duke of Devonshire following his marriage in 1748 to Lady Charlotte Boyle.  The sixth Duke (1790-1858) undertook the most extensive restoration.  He was a patron of Dickens, Thackeray and, most importantly, Joseph Paxton who was the under-gardener in 1823 and became the Duke's friend.  Botanist, inventor, engineer, architect, Joseph Paxton designed the Crystal Palace for the London exhibition of 1851.  He then became a Liberal member of Parliament.  Tsar Nicholas I of Russia knighted him in 1844 and he was later knighted by Queen Victoria.  It was he who played a leading part in creating Lismore Castle as it is today.

During the renovation in 1814, a valuable manuscript now known as the 'Book of Lismore' and the 'Lismore Crozier' were found in an excellent state of preservation. They are now in the National Museum in Dublin.

Lismore Castle has been involved in Ireland's cultural, social and political history since its creation. It was attacked and besieged three times during the civil wars of 1641-1649.