Ardfinnan Castle, Norman castle on River Suir crossing, Ardfinnan, Ireland

Ardfinnan Castle is a parallelogram-shaped Anglo-Norman castle on a rocky incline above a ford of the River Suir. Its fortified entrance gateway, keep and a circular 13th-century preceptory stand at the eastern corners, while two ruined towers and a great hall lie to the west. Trees of Castle Wood hide much of the structure, and an old watermill bounds the site at the riverbank, bridge and road.

Prince John of England built Ardfinnan Castle in 1185, on the site of Ardfinnan Abbey, during his first expedition to Ireland; with Lismore Castle it is among the oldest castles raised by the English Crown in Ireland. Maurice de Prendergast served as its governor, and John issued royal charters there, one signed "King of Limerick". The Irish king Donal O'Brien drew the garrison out by a feigned retreat and took the castle that year. The Knights Templar built its round preceptory tower in the late 12th or early 13th century, and the site later passed to the Knights Hospitaller.

The Irish name Ard Fhíonáin is anglicised as Ardfinnan. The site was an early-Christian monastic centre on Saint Declán's 5th-century pilgrim path, developed into an abbey by Saint Mochuda around 632 AD; the castle was built over that former abbey. Tradition credits the Knights Templar with founding Ardfinnan's woollen mills in the 12th century, with a fulling mill near the castle.

The castle is a private residence with no public access, but it remains visible from outside near Ardfinnan village center in County Tipperary. Visitors can view the exterior and structure from the village perimeter without entering the grounds.

Ardfinnan and its sister Lismore Castle, built in 1185, are considered the oldest castles erected by the Norman Crown in Ireland. John issued royal charters while staying at Ardfinnan, and one was signed "King of Limerick", possibly the first time an English crown figure claimed a kingship in Ireland.