Passage Graves are Celtic graves, set in a mound of earth or stone with a passage leading to the central chamber, and often have side chambers. Many are 4,500 years old and show a sophisticated knowledge of construction, design and astronomy. They are often decorated with geometrical motifs, spirals, concentric circles, triangles, zigzags, the human face and, of course, the sun. Their meaning has not yet been deciphered but presumably they are connected with the religion of the people who built them.

Passage graves often occur in groups and those found in the Boyne Valley are superb: Newgrange, Knowth and Dowth.