The Portal Tomb of Birrinagh (in Irish, Biorainneach), built between 4000 and 2500 BC during the Neolithic period, is located on flat land, approximately 90.0 meters southeast of a small river that flows into Lough Gowna (also known as Loch Gamhna, meaning “Calf Lake”). It is situated about 1.5 kilometers southeast of Moyne in the northern part of County Longford, Ireland.

Portal tombs are megalithic structures found across the British Isles, characterized by two equally tall, upright stones with a door stone in between, forming the front of a chamber covered by a partially massive capstone.

The small Portal Tomb is approximately 1.5 meters in length and consists of two portal stones, each standing at about 1.8 meters in height, with a door stone standing at 1.5 meters tall in between them. A stone, standing at 1.3 meters in height, leans against the northern portal stone on the western side. This stone is not an endstone but rather the capstone, which was moved in the 1960s, measuring approximately 2.0 meters in length, 1.55 meters in width, and 0.4 to 0.2 meters in thickness. According to local information, the side stones were removed around the same time.