These structures, which belong to a tradition of Neolithic communal burial dating back roughly five thousand years, are more commonly associated with dramatic coastal headlands or upland ridges. The one at Birrinagh is quiet and unassuming, and it is also incomplete in ways that are only partly explained by the passage of time.

Portal tombs are defined by their paired upright portal-stones at the entrance, which frame a doorstone and give the whole structure a distinctive, almost architectural formality. At Birrinagh, both portal-stones stand 1.8 metres high, flanking a doorstone of 1.5 metres at the east. A backstone, 1.3 metres high, stands 1.5 metres to the west of the doorstone, marking the far end of what was a small chamber, just 1.5 metres in length. The roofstone, measuring 2 metres long and up to 0.4 metres thick, has slipped from its position and now leans against the north side of the structure rather than covering it. What makes Birrinagh particularly interesting is not only what remains but what was taken. Local tradition holds that a sidestone from the south face and what may have been a secondary roofstone were removed during the 1960s, which would account for the chamber's current lack of sidestones. A low earthen mound surrounds the site, though this appears to be largely a recent feature rather than any original cairn material, according to Ó Nualláin's 1989 study of the monument.