The mound or tumulus is about 280 ft in diameter and 150 ft high and surrounded by large kerbstones, some of which are decorated. Quartz was found fallen outside the kerbing, suggesting that the entrance to this tomb was surrounding by glittering white, as at Newgrange.
Three stone-lined passages lead into the mound from the west. The long passage is crossed by 3 sill-stones and ends in a cruciform chamber with a lintelled roof. Several of the upright stones of the passage and chamber are decorated with spirals, chevrons, lozenges and rayed circles.
Dowth shares a special solar celebration with neighboring Newgrange during the winter solstice.
Martin Brennan, author of The Stars and the Stones: Ancient Art and Astronomy in Ireland discovered the remarkable alignment during the course of his ten-year study in the Boyne Valley. From November to February the rays of the evening sun reach into the passage and then the chamber of Dowth South.
During the winter solstice the light of the low sun moves along the left side of the passage, then into the circular chamber, where three stones are lit up by the sun. The convex central stone reflects the sunlight in to a dark recess, lighting up the decorated stones there. The rays then recede slowly along the right side of the passage and after about two hours the sun withdraws from Dowth South.