Dunmoe Castle, Medieval castle ruins near Navan, Ireland

Dunmoe Castle is a medieval fortress that retains two of its original circular towers, rising above the River Boyne between Navan and Slane. The four-story rectangular structure displays arrow slits in the south wall and a vaulted ground floor.

The D'Arcy family built this fortress in the 15th century to mark the western boundary of the English Pale. A fire in 1798 destroyed much of the structure and two of its four towers.

The name likely comes from Irish Dún mBó, "hillfort of cattle," suggesting a Gaelic fort stood here before the castle. A couplet recalls George d'Arcy hosting King James the night before the Battle of the Boyne in 1690 and King William the day after: "Who will be king, I do not know, But I'll be d'Arcy of Dunmoe."

The ruins stand on the northwest bank of the Boyne, 4 km northeast of Navan, on a grassy slope reached from the road; the ground is uneven and the castle has no roof or floors, so watch your footing. The ground floor is buried and the surviving towers are not accessible inside.

The castle's Irish name, Dún mBó, means "hillfort of cattle", suggesting a Gaelic fort stood here before the D'Arcy castle. The Civil Survey of 1654–6 recorded a castle, church, mill, orchard and fishing weir on the Darcy estate. The newel stairs in the surviving southwest tower include a gun loop.