Slieve Croob may have been the mountain named Brí Erigi or Brí Airige in medieval writings. The cairn on its summit is believed to be the remains of an ancient burial mound, possibly of a passage tomb like the one on Slieve Gullion. In the 19th century it was recorded to be around and in conical height, with forty-two pillar stones or kerbstones around the edge. The cairn would have had a well-defined shape when it was built, but over time it has slipped and been damaged by visitors. Irish folklore holds that it is bad luck to damage such cairns. Some of its stones have been piled into smaller cairns on top of it, which led to the summit being nicknamed 'The Twelve Cairns'. Traditionally, people would gather on the summit at Lughnasadh where they would add a stone to one of the cairns. They would collect and eat bilberries and there would be folk music, dancing and games.