Purple Mountain's individual prominence qualifies it as a Marilyn, and its prominence is within 10 metres of the P600 prominence threshold of , which classes Purple Mountain as a Sub-major mountain. Purple Mountain meets the Arderin, Simm and Hewitt classifications. It ranks as the 12th-highest mountain in Ireland on the MountainViews Online Database, 100 Highest Irish Mountains, where the prominence threshold is 100 metres.