Ballybrittas portal tomb

In a portal tomb, two upright stones of equal height with a door stone between them form the front of a chamber that is usually covered by a capstone, sometimes of great size. At Ballybrittas the capstone has an area of 3.6 m² and rests at the front on portal stones 1.8 m high. The large interior is partly closed by a half-height door slab, now broken and about one metre high.

A portal tomb, or portal dolmen, is the Irish and British version of the dolmen: a single-chamber megalithic tomb with two or more upright stones framing a portal at the front and a large capstone. In the Irish classification set out by Seán Ó Nualláin and Rúaidhrí de Valera in 1961, portal dolmens are one of four categories, alongside court cairns, wedge-shaped gallery graves and passage tombs. Most portal tombs date from the Neolithic, between 4000 and 3000 BCE, and some were covered by a mound of earth or smaller stones that has since eroded away, leaving the stone skeleton.