Mannan Castle is a motte-and-bailey and National Monument located in Donaghmoyne, Townland Tullynacross, on site of the Mannan Castle Golf Club, in County Monaghan, Ireland.

[3] This site is believed to have been the ancient capital of Airgíalla (Oriel), a kingdom originating sometime in the pre-Christian era (according to semi-mythical accounts, it was founded by The Three Collas in AD 331).

This site was not necessarily chosen for its military or defensive usefulness but for reasons of prestige.

By building atop an ancient Gaelic Irish capital, the Norman conquest of Ireland was made clear.

Various local legends attach to the castle; one claims that the mortar was made with animal blood and this is why a stone cannot be dislodged from the wall.