To guard the northern border of Waterford, John's father Henry II of England proposed Ardfinnan and Tybroughney on the river Suir, with Lismore on the Blackwater as key positions to erect castles.

Indicating retreat, he turned back only to be pursued by the small garrison of knights holding Ardfinnan Castle, which for O'Brien would play the advantage.

After this and further successive defeats against the Irish Kings, John's original force of 300 men was decimated, and by December of that same year, 1185, he was summoned back to England by his father.

With cannons placed on a hill opposite the castle, Ireton bombarded its once impenetrable walls until there was a large breakthrough after about 8 shots and then proceeded to kill about thirteen of the out-guard and lost only two of his men with about ten wounded.

Despite being in ruins, the position of the castle still commanded over a chief pass on the river Suir and it would be used along with the rest of the Ardfinnan and Neddans area to hold a British Army summer training camp, with reserves ready against French invasion.

Although initially established as a temporary encampment for the summer months, it became a permanent camp in March 1796 by the orders of John Pratt, 1st Marquess Camden, which amounted a force of 2,740 mainly Protestant soldiers.

[1] The last male member of this family to occupy the castle was Royal Navy Admiral, Sir Robert Prendergast, who settled in Eastbourne following his retirement in 1920.