The Carraig á Mhaistin stone (transl. Bully Rock) is a megalithic tomb located in Cork harbour. It was previously believed to have been an 18th-century folly, built by Murrough O'Brien, 1st Marquess of Thomond, like the nearby Siddons Tower folly.

The monument was first reported by antiquarian John Windele after an 1860 visit to the area. A man named Dr. Wise had the capstone raised from where it had fallen.

The tomb is nearly submerged at the high tide mark, as is the accompanying cairn. The tomb is made of three upright stones largely covered in seaweed and a capstone. It faces east.

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• Cott, Grace M., Marcel AK Jansen, and Deborah V. Chapman. "Salt-Marshes on peat substrate: where blanket bogs encounter the marine environment." Journal of Coastal Research 28.3 (2012): 700-706. " In other parts of the country, O'Kelly (1945) referred to the Rostellan Dolmen in county Cork, estimated age 4000 BP, which becomes partially submerged at high tide. " doi:10.2112/JCOASTRES-D-11-00108.1
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• Viney, Michael (2014). Ireland: A Smithsonian Natural History. Smithsonian Institution. ISBN 9781588344243. Robert Lloyd Praeger with his wife, Hedwig, standing on Rostellan Cromlech, a megalithic tomb, during a fieldclub conference in 1907

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