Sliabh an Iarainn is an important natural heritage site due to unbroken sequence of Carboniferous marine fossils present in the rock layers spanning the Namurian (326-315 million years ago) and lower Westphalian (313-304 million years ago) stages of the Silesian (series). The Geological survey of Ireland (1878) wrote the Geologist may examine all the formations of the district from the Lower Silurian up to the outlier of Coal-measures that crowns Slieve-an-Ierin… It is a rare thing in most countries to find so much comprised in so small a space.