This is the name given to a range of sedimentary rocks, many of them sandstones, conglomerates and mudstones. They are usually red or purple in colour and formed in semi-arid, non-marine, conditions during the Devonian period.

In Ireland, thick sequences of various types of Old Red Sandstone are extensively represented in Munster where they form the highest mountains.

Old Red Sandstone can be contrasted with the much younger New Red Sandstone dating from the Triassic period. In Ireland, rocks of this period are mainly represented in parts of the north-east.