Megalithic tombs, built during the Neolithic period roughly five thousand years ago, were communal burial monuments constructed from large upright stones capped with a heavy horizontal slab. They survive in various forms across Ireland, including court tombs, portal tombs, and passage tombs, and Mayo is particularly well furnished with them, making any single example easy to overlook.

The tomb at Gleann Chaisil was documented by Ruaidhrí de Valera and Seán Ó Nualláin as part of their county-by-county Survey of the Megalithic Tombs of Ireland, the Mayo volume of which was published in Dublin in 1964. De Valera and Ó Nualláin spent years travelling the country recording these monuments with considerable rigour, and their survey remains a foundational reference for understanding the distribution and condition of prehistoric burial architecture across the island. That a site appears in their work at all is a mark that something worth recording survived, even if the condition of individual tombs they catalogued varied widely, from well-preserved chambers to little more than a scatter of fallen orthostats.