Killarney National Park is underlain by two distinct and contrasting rock formations. A major geological boundary runs across the National Park, generally from north-west to south-east, mostly beneath the waters of Lough Leane and Muckross Lake. It separates a mountainous area of Devonian rocks to south and west from a plain underlain by Carboniferous rocks to north and east.
The Devonian rocks, generally referred to as Old Red Sandstone, consist of numerous layers of sandstone interbedded with grits, slates and conglomerates. The oldest Devonian rock formations, exposed in the valley of the Upper Lake, are predominantly green in colour, while the succeeding formations are generally red, purple or grey. The proportion of fine-grained rocks among the hard erosion-resistant sandstones also tends to increase from older to younger.
The Devonian bedrock outcrops very extensively at the surface, and is only very locally covered by deep glacial deposits, soils or peats.
The Carboniferous rocks are predominantly limestone, including both massive unbedded reef limestone formations and bedded calcarenites. There are also relatively thin but significant chert (i.e. siliceous) formations, comprising layered and nodular black cherts among finely stratified limestones and calcareous mudstones. The Carboniferous formations outcrop extensively along and near the Muckross Lake and Lough Leane shores, the outcrops including expanses of irregular limestone pavement with numerous vertical fissures.
Away from the lakes the Carboniferous rocks are generally covered by thick glacial deposits. One of the chert formations has been partially altered to form the hard white and pink Killarney marble, which outcrops on the Muckross Peninsula and elsewhere, and was occasionally quarried in the past for local building use. A considerable amount of mineralisation also occurred in some Carboniferous strata, with the formation of copper minerals which were mined in prehistoric times and again at Ross Island and at Muckross in the 18th century.
For the most part the Devonian/Carboniferous boundary near Killarney is marked by the Millstreet – Muckross Fault Line, an enormous thrust fault which raised the lower older Old Red Sandstone beds high above the younger Carboniferous limestone. However, transitional formations from Devonian to Carboniferous do outcrop on the Muckross Peninsula. These consist of grey sandstones and laminated black siltstones and claystones.