The Geological Survey of Ireland (GSI) describe Lugnaquilla as a slate capped, granite rooted, relatively flat-topped mountain. Crags of dark-grey schist protrude from the upper cliff walls of Lugnaquilla's corries which are Ordovician in age. The protrusions of lighter grey rock are granite. The cap of schist overlying Lugnaquilla's granite core is the remnant roof of the magma chamber into which the Lugnaquilla granites were emplaced. Cosmogenic dating on exposed bedrock showed that these schists were not covered by ice during the Last Glacial Maximum, and thus Lugnaquilla was a nunatak.