The castle came into the possession of John Power of Dunhill in 1641, though not through any grand conquest or inheritance; he acquired it when David Power from Shangarry in County Cork defaulted on a mortgage. Before the Powers held sway here, the castle may have belonged to the Prendergast family during the 16th century, though historical records from this period remain somewhat uncertain.

Today, visitors to the site will find only the foundations of what was once a substantial rectangular structure, measuring roughly 13 metres north to south and 12 metres east to west. These weathered remains still command an impressive position, overlooking Newcastle parish church about 70 metres to the south-southeast. The strategic placement of the castle on this elevated ground would have given its inhabitants clear views across the surrounding countryside, a crucial advantage in those turbulent times.

The site represents one of many fortified houses and castles that once dotted the Irish landscape, serving as both homes and defensive positions for the Anglo-Norman and Old English families who controlled much of the country's wealth and land. Though little remains of Newcastle Castle today, its foundations serve as a tangible link to Waterford's medieval past and the complex web of family allegiances, financial dealings, and property transfers that shaped the region's history.