Sites include Sithean Dubh, a chambered cairn where in 1831 it was said two skeletons of "gigantic size" were found.
BBC: Website reveals hidden landscape in Caithness
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The 23-year-old man suffered stab wounds to his stomach at his home in Sliabh Dubh View just before 04:00 BST.
BBC: Ballymurphy area of west Belfast
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The tidal basin formed by the two converging rivers was named "dubh linn" (meaning "black pool") by Celts, who had inhabited the area upstream as early as 500 BC.