The cairn is 28 metres in diameter and rises to about 4 metres, and it is not alone up here. Two further passage tombs lie close by, one just 7 metres to the north-east and another to the south-west, making this a genuine prehistoric complex strung along the ridge rather than a solitary monument.

Passage tombs are megalithic burial monuments, typically Neolithic in date, in which a stone-lined corridor leads to a central chamber, the whole structure covered by a mound or cairn. This particular cairn has no visible kerbstones, the upright stones that would originally have defined its edge, but a ledge running around the base of the mound indicates that they are still there, buried beneath accumulated material. The mountain itself carries a telling name. The Ordnance Survey Letters of 1837 recorded that local people knew it as Suighchan, meaning the Seat, a name that suggests the summit held some significance in folk memory long after the original purpose of its monuments had been forgotten. Scholars including Healy and Herity documented the site from the 1960s onwards, and more recent survey work by Redmond and Mac Aonghusa in 1994 has added to our understanding of the cluster.

Seahan Mountain sits in the Ballinascorney area of south County Dublin, within reach of the city but firmly in upland terrain. The approach is on foot across open moorland, and the ground can be soft in wet weather, so appropriate footwear is worth thinking about. Once at the cairn, the view takes in the Liffey valley to the north and, on a clear day, the passage tomb complex at Mountpelier to the east, a reminder that Neolithic communities were choosing these elevated locations with some consistency. The trigonometrical station on the summit makes the cairn easy to identify, though it does take a moment to look past the concrete pillar and register the prehistoric mass beneath it.