Ballymoon Castle, Medieval ruin castle east of Muine Bheag, Ireland

Ballymoon Castle is a ruined square courtyard about 80 feet (24 m) on each side, enclosed by granite walls 20 feet (6.1 m) high and 8 feet (2.4 m) thick at the base. The west wall has an arched gateway with portcullis grooves, and cross-shaped gun loops and arrow slits pierce the walls. Foundations mark doors and a large double fireplace of the great hall on the north side.

Ballymoon Castle is thought to have been built in the 13th or early 14th century, either by Roger Bigod, 5th Earl of Norfolk, or by the Carew family, who acquired the land after Bigod died without issue. Much of its history has been lost. In the late 1800s it was bought by Michael Sheill from Wexford, who established local businesses.

The Irish name Caisleán Bhaile Muáin pairs the castle with the settlement of Baile Muáin, the townland that gives the ruin its name. The site stands in a field beside the Fennagh road, and the surrounding townland is still called Ballymoon today.

Ballymoon Castle stands in a field beside the Fennagh road, about 2 miles (3 km) east of Bagenalstown. A small wooden bridge over a ditch gives access to the site. Visitors can walk the walls at ground level; the interior courtyard is open and the ground uneven.

The west wall's arched gateway still shows the grooves where a portcullis once dropped, and a barbican may have stood in front of it. Cross-shaped gun loops and arrow slits are cut into the granite walls, details easy to miss at a glance.