The mountain was shaped by the southwestward movement of ice age glaciers over millions of years, the morainic drift heaping thousands of drumlins in the surrounding lowlands.

[9] 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".

[11][n 1] She described some rock layers as particularly fossiliferous,[12] the shale bands abundant with goniatite faunas and Bivalvia marine and freshwater molluscs.

[14] The richest and most diverse band in the succession at Sliabh an Iarainn, in terms of species present, contains Trilobites, brachiopods, gastropods, echinoids and Bryozoa.

[16] Overall, Yates documented nearly 120 distinct fossiliferous sites around Sliabh an Iarainn, her work complemented by extensive photographs of often beautifully preserved fossils.

[21] Impressive landslides have occurred along the western face, and at the south-western and south-eastern corners of the mountain, indicating an appreciative magnitude of land-slipping.

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.

[27][28][26] The Sliabh an Iarainn project is a literary initiative started in 2004 to write about the history of the people who inhabited the necklace of townlands on the flanks of Sliabh-an-Iarainn and Ben Croy, in County Leitrim.

The goal was to preserve a memory of the Ultachs, Catholic refugees displaced out of Ulster in 1795 who made a home on the mountain, their experiences of famine and emigration, and the resilience of the remaining communities.

[29] This social history was released in three volumes- In the 1609 Plantation of Ulster, Sliabh an Iarainn formed part of lands which were granted to John Sandford of Castle Doe, County Donegal (the father-in-law of Thomas Guyllym of Ballyconnell) by letters patent dated 7 July 1613 (Pat.

[33] The siting of Smelting works contiguous to Lough Allen allowed for the transportation of Pig Iron in boats of up to forty tons.

[38] Local tradition says "about the year 1650, there was a furnace for smelting Iron ore in the downland of Cornashameogue, situated on the east side of Lough Allen.

[39] Local folklore recalls a so-called "Sliabh an Iarainn canal" connected with Cornashamsoge smelting works- "the ore had to be conveyed to the furnace for a distance of about 3 miles.

[41] There was an Iron works at Swanlinbar in County Cavan, right at the far north-east corner of Sliabh an Iarainn,[42] though it had closed by 1785 according to an observer who wrote- "The furnaces of Ireland were never so forsaken and deplorable a way as they are at present...

[n 5] On the eastern side of Sliabh an Iarainn there is another abandoned level in the upper seam which is 0.3 metres (11.8 in) thick, the location possibly being above Aughacashel House.

[46] These great forests in Leitrim and on the west side of Lough Allen were denuded for the making for Charcoal for Iron works around Sliabh an Iarainn.

[45] The Book of Invasions describes the Tuatha Dé Danann, tribe of the goddess Danu arriving in Mesolithic Ireland through the air before landing their floating-ships on the summit of Sliabh an Iarainn, "the mountain of Conmaicne Rein in Connacht".

Metal workers were held in high esteem, and the Irish Pantheon Gobán Saor is synonymous with the legendary Scandinavian named Vaeland Smith and Goibniu of the Tuatha De Dannan.

[53][54][55] Some fringe historians suggest a passage in the Book of Invasions concerning the appearance of the Tuatha Dé Danann in Ireland, records "the arrival of aliens in spacecraft with cloaking devices" at Sliabh an Iarainn.

[56][n 7] In the parish of Kiltubrid the term fear gorta (Irish for "hungry man") refers to a hunger which may supposedly afflict a person on the mountains, proving fatal if not quickly satisfied.