Sliabh an Iarainn is the most eastern part of the Connacht coal field. Well-marked escarpment lines are visible, partly exposed by lines of geological fault on all sides of the mountain valleys, the collapsed layers removed by denudation. The outcrop of two coal seams, crow coals with a sandstone roof and middle coal under a slate roof, are traceable some difficulty along the grit escarpment on the western side of Sliabh an Iarainn towards the Stony River valley, becoming completely obscured by drift deposits on the southern flanks, and on the eastern flanks to a mile North of Lough Nabellbeg continuing through the townlands of Sradrinagh and Cornamucklagh South obscured by a thick blanket of peat bog, becoming visible again further north on the western side of the hill at Cleighran More and Cleighran Beg where faults are evident. The outcrop of both coal seams is also traceable for along the south-eastern slopes of Bencroy. More than two coal seams may be present at Sliabh an Iarainn, though the only rocks observable over the coal seams (in the millstone grit) are the lower coal measure containing black and brown splintery shales of a considerable thickness at Bencroy to the east and Barnameenagh to the west.