Greenmount Motte & Bailey is located in County Louth. Today it appears as a large mound in a field.
This is the earliest form of Castle in Ireland. Motte & Bailey Castles first appeared in the 1100's and consist of an earthen ringwork, a courtyard called a Bailey, and a mound called a Motte. A wooden structure would have stood atop the Motte.
This particular example was once known as Droim Catha, meaning Battle Ridge. The name points to the defensive nature of the site.
Though many Motte & Bailey Castles were repurposed for later stone structures, this was not the case with Greenmount, though it's defensive nature was taken advantage of once again five centuries after its construction when the Catholic forces used it as a base during the 1641 Rebellion.
A drawing of Greenmount Motte & Bailey from 1748
References
National Monuments Survey | Ireland | VIEW SOURCE |