This is one of the least-known monuments in the Hill of Slane complex despite its impressive dimensions. The monument is unfortunately almost completely obscured by trees planted across the summit of the hill during the eighteenth century and when in full leaf, the features on the western side of the hill are almost completely invisible from the College and Graveyard.
The mound is classified as a motte, an earthen hill created by upcast soil from a large ditch, part of an Anglo-Norman castle. Slane was the heart of a medieval barony granted to the le Fleming family and contained a castle, church and a borough settlement. Richard le Fleming built castles at the power centres of Knowth (Cnogba) and Dumhach Sláine. We know from the song of Dermot and the Earl that this was ‘Un mote’ – a motte castle – at Slane. The castle, dwelling and garrison were destroyed in 1176 by Maol Sheachlainn Ó Lochlainn, king of Cinéal Eóghain with the reported deaths of five hundred people. The word Dumhach itself is usually refers to a burial mound and the mound has characteristics of other local enclosed barrows (see below). Within the metrical Dindsenchas, a poem written in the 12th century to explain Slane’s placename, we learn that Sláine, king and judge of the Fir Bolg, died and was buried on Druim Fuar in a great mound called Dumha Slaine (CELT 2005, Poem 77). This story links the mound to themes of kingship and judgement. Thus, it seems certain that the first castle at Slane was built overlying an important prehistoric site close to the medieval parish church.
Although it is now hidden and overgrown this monument is impressive and was originally designed to dominate the local landscape as a centre of military and administrative power. It would almost definitely not have been surrounded or covered by trees and would have been visible over a large area.
The monument was surveyed in 2010 by the Hill of Slane Archaeological Project. It is steep-sided and measures 7.8m high with a summit measuring 20m N-S by 23m E-W. Low stone walling is evident around the summit edge, especially on the N side. The motte is c.45m wide at the base and is surrounded by a 4-5m wide ditch up to 2m deep, partly rock cut, especially along the SE side. The motte stands centrally within a circular enclosure c.163m in diameter. This is well-defined along the S and SW side as a bank with an outer ditch, but this gives way from the NW to the NE side to a simple terrace while to the E and SE it is a low bank. This is not a feature of a classic Anglo-Norman motte-and-bailey and is likely to significantly predate it. Low earthworks lie within the enclosure on the E side of the motte. Given the very prominent location of the motte, the proximity to the Brú na Bóinne complex and the reference to an earlier burial mound at the site in the medieval source, it has been suggested that the motte may be constructed on an earlier, possibly prehistoric mound. The Anglo-Normans are known to have remodelled existing mounds when constructing their mottes firstly because this reduces the overall amount of work they had to do to create a stronghold and secondly because these were often seen as important sites in the landscape. This happened with both mounds at Knowth and Dowth in the Brú na Bóinne complex and may very well have happened at the Hill of Slane as well. The 2010 geophysical survey on the motte aimed to explore this possibility and results of the surveys suggest a buried stone structure and the circular enclosure surrounding it has similarities to a prehistoric barrow and enclosure at Mountfortescue 5km to the north.