Benbulbin, and the Dartry Mountains as a whole, are composed of limestones on top of mudstones. These rocks formed in the area approximately 320 million years ago in a shallow sea. Uppermost in the limestone layer is a thicker, harder limestone called the Dartry Limestone Formation. Below this is a thinner transitional limestone formation - the Glencar Limestone Formation. Further down, the lower slopes consist of shaly mudstone known as the Benbulben Shale Formation. Scree deposits are found near the base.