These are Megalithic tombs of around the forth and third millennium B.C.
Often located on a hilltop, the graves have a covered passage leading to a tomb at the centre of a mound.
Those in the Boyne Valley……notably Newgrange , and Knowth-and Loughcrew in County Meath, are the most decorated.
Other passage- graves are found at Carrowmore and Dowth, County Louth.
Worth seeing on an Ireland tours.
