On the western downslope of Seahan Mountain in Ballinascorney Upper, County Dublin, a low circular mound of gravel and stone, about eight metres across and barely half a metre high, conceals the disturbed remains of a passage tomb. It is easy to mistake for a natural feature of the terrain, which is partly what makes it worth knowing about.

Passage tombs are among the oldest monument types in Ireland, typically consisting of a stone-lined corridor leading into one or more chambers, all covered by a mound of earth or rubble. The structure beneath this mound has been interpreted as a possible cruciform chamber, meaning it may once have had a cross-shaped interior with a central space and side recesses, a layout familiar from grander examples like Newgrange. The passage appears to have been aligned roughly north-northwest to south-southeast. As documented by Redmond and Mac Aonghusa in 1994 and referenced earlier by Herity in 1974, the stonework is now disturbed, so much of what the tomb once looked like can only be inferred. The quartz block near the possible entrance is a small but telling detail; quartz was frequently used at Irish passage tombs in ways that suggest it carried deliberate symbolic or ritual significance, though its precise meaning here, as elsewhere, remains unknown.

Seahan Mountain sits within the Dublin Mountains, and reaching this monument requires a walk across open upland ground. The mound is on the western downslope of the summit rather than at the top itself, so it can be easy to pass by without noticing it. The low profile of the cairn means it reads better in raking light, particularly in winter or on overcast days when shadows define what little remains of the stone structure. There is no formal path to the site, and the ground can be boggy depending on recent weather. The quartz block, if still in position, is the clearest visual marker at close range.